x axis skipping

I'm about to go mantel from this issue. I've spent the entire day (literately a solid 18 hours) trying to resolve this. Small prints come out perfect. It seems anything that makes my x axis move more then 20 mm will get a skip. I've moved the x carriage about and I can feel the motor jam and jump at varying places. I'm using a prusa i2 with gen 6 running marlin.

And what I had tried:
-verified step calibration
-verified firmware speeds (dropping acceleration)
-greased the x axis
-ensured the belt was centered
-ensured the blet was clean
-ensured the gear on the motor was tight
-ensured teeth on belt and gear were intact
-adjusted pot to idel area (and sums unideal)

ANY help would be greatly appreciated!

Do you have a bowden extruder, or mounted on the x-axis? I had one printer skip because the bowden sheath was getting snagged by the filament being wound (near the end of the spool) tighter than the radius of the sheath causing skips.

Is your hotend hitting anything when it makes moves and does it skip when printing or on travel moves? If your x-axis isn't tightly resting on the nuts for the z-axis it could hit something and pop up a bit and skip.

I actually have a wades extruder (figure I was forgetting something!)

As for the hotend, it doesn't. That was my third thought, so I was watching that like a hawk. the only time it does drag is if a skip happens, and I let i print. As for when it skips, it's all over the place. Sometimes it will skip on a travel step, other times it will skip on a print step.

Have you tried printing slower?

Yup. I first started a lower travel, then I went to just slower everything. Still skipped

However, this morning I decided to loosen the belt to take it off and take a closer look at it (and as I thought while it was still on, it was fine) but then stuck it back on, tried printing jsut for kicks and giggles, and hey presto: it's not skipping.

Any reason why adjusting the belt tension would cause this? Or is one of those "voodoo magic fixed it" moments?

I've had that same thing happen before and I think my belt was hitting my guide washer on the idler x-axis causing too much friction.

I also think there is some sweet spot for belt tension depending on....just about everything else in the setup. I know I have done everything, including changing the belt, to only have the problem stop when I moved the zip-tie that tensions my belt to the other end of the tensioner.

I would say "sounds good" but I'm always uneasy when the issue is resolved by "its now tighter" (not to say I'm unappreciative for the feedback!). However, I'm lucky to have a tensioner that is set up via a screw that is pretty easy to get to.

Have you checked your wires, might be a faulty connection?

I did. Nothing is pinching, nothing is streched, and there getting good connections.

And I spoke too soon. the skipping is back again...

So the skipping of the x axis sort of comes and goes...

It seems that when it does come, I can solve it by greasing the rods of the x axis. This seems to solve it for about 5 hours printing time. I'm not too familiar with linear bearings, but my gut seems to want to say I might need to repack them with grease (again, not familiar with linear bearings, so I don't even know if they need repacking from time to time).

I also thought that *maybe* the axis was jamming somewhere (as if I have the motor off, and I move the carriage, I can feel it stick and jump at random spots). Running off this, I have watched prints like a hawk, waiting for a skip, and all I see is what looks like the layer shifting on purpose (as if the gcode told it to do just that).

Also, odd thing I've noticed...it only seems to happen on new layers. For example, layers 1-5 print fine, and layer 6 is shifted.

Just took the belt off my x axis, and the carriage moves freely (I could sneeze, and the thing would roll).

However, the moment I get the belt back on, that jamming I can feel comes back. I have taken various steps of belt tension, and regardless how tight or loose I have belt, I still have that issue with the binding. I have checked both the belt and the gear, and all the teeth are accounted for.

How is the belt aligned on the idler/gears? Check for any possible rubbing of spots where it could snag. I had this issue where I could not possibly fix it and still haven't figured it out exactly.
I use this [www.thingiverse.com] to help make sure everything is tight down on the z-axis when the carriage is moving which helped and ended up having to use a large desk fan to cool the parts enough. My main issue was when I would print a M8 bolt hole the converging walls (top half of the bolt hole) would curl up and the nozzle would hit.

I've tried lubricating, tensioning etc, with no luck other than printing and praying.
I've experienced this on my printer using a J-head and a derivative of this x-carriage (http://www.thingiverse.com/thing:45883). I haven't experienced it with the 2 printers I have with budaschenozzles and carriages with 3-bearings and zip tied bearings.

How much current? What is your power supply?

BenWittbrodt: everything was (and still is) dead straight. No rubbing anywhere. I'm not too sure how the first part works, but I am curious. And that second link is interesting as well! May be something I will look at at a later date.

Soldanr: no idea. I have yet to check. I might, but right now, everything is working again. As for power supply, it's a standard ATX

And as stated, it fixed itself. Again. I was still messing around with the belt tension, moved the carriage randomly: no binding. I'm clueless as to what caused it, but at this point, I'm just counting my blessings its working.

Also I just found out the hard way that when trying to microstep (ex: 1/16th on a Polu) with a high voltage stepper things can get inconsistent.
I had issues of X axis and Y axis skipping in long moves. I was using a high voltage and low current stepper. It only happened a few times during some random print jobs. Finally I got the right stepper motors (low voltage, high current) and it works like a charm.

From Sanguino's Wiki:
First of all, note that there are usually two types of NEMA 17 motors :

high voltage stepper motors, that work usually on 12 to 14V, the working current is usually below 1A. These don't work well with microstepping chopper drivers and are not recommended.
low voltage stepper motors, that work usually on 2 to 4V, the rated current is usually over 1A.

It is safe to drive low voltage stepper motors at a much higher voltage because the Pololu A4988 has current limit functionality. The higher the voltage applied compared to the motor's rated one, the faster your stepper motor can run. The A4988 chip can only provide up to 2A per coil so choose your stepper motor accordingly.