J-head uneven flow

I have a problem with my J-head 0.5mm, the flow of plastic is not even but comes in pulses, rather long pulses but very detrimental for the print quality. Sometimes the flow is normal for around 20s and after that stops completely for 5-10s...
When I extrude with pronterface there is a pause before the plastic starts to flow and during the extrusion as well.
Any ideas? I have tried to raise the temperature to 190 and 200 degrees but that did not help.

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Prusa with cast PU parts, RAMPS 1.3, Wades extruder, J-Head hot end

I have the same issue.. I have only noticed this behavior with certain filaments. I had some issues with the red and atomic green from reprapworld.com
Is it the same as yours?

My red PLA is from reprapworld.com so that could be the cause of this, but I also have a clear PLA purchased from 2printbeta.de with the same issue. Have you tried to dry out your filament? I will try to dry a meter of each filament in the oven tonight and see if that solves the problem.
I store my filament spools in a sealed container and with a desiccant and the winter climate is really dry indoors right now, around 20-25% humidity.

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Prusa with cast PU parts, RAMPS 1.3, Wades extruder, J-Head hot end

I haven't tried drying the filament. Although my spools are just stored in a cardboard box. I have been using other filament for a while without the problems (from grrf.de). Also a single spool of clear PLA from a danish supplier, which has been printing without issues.

(Original Mendel, Sprinter and J-Head)

Drying the filament did not help at all.
What I have noticed is that when I remove the filament from the hotend the filament ends with a distinct bulb, see attached photo.
I think that there is a gap between the brass and the PTFE, will disassemble my J-head to investigate.

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Prusa with cast PU parts, RAMPS 1.3, Wades extruder, J-Head hot end

do you have a fan on the PEEK insulator? It looks like the transistion is creeping up the filament. I dont have a j-head so just a gess looking at the pic. Good idea to take it apart and have a look.

Yes, I have a fan directed at the PEEK part of the J-head.
What I have noticed is that when the flow is slow there is always a string of bubbles in the cleas PLA coming out of the nozzle, when the bubbles stop the flow returns to normal again. Could this be a sign of moisture in the PLA? Maybe I should try to dry the filament for longer and slightly hotter, last time I dried it for 2h in ca 50 degrees celsius.

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Prusa with cast PU parts, RAMPS 1.3, Wades extruder, J-Head hot end

Have you checked you Hobbed bolt for clogging? Sometimes a small bit of plastic can break off and fill in the gap between teeth. When that happens it will extrude ok for most of the time, then when it hits the clog it will just slip on the filament and you will get a pause in extrusion. Looking at the pic you posted there are some periodic spots on your filament that Appear to be without teeth marks.

I will check my hobbed bolt, but right now it is printing just fine after the filament spent several hours in my food dehydrator at 60 degrees C.
Seems to have been moisture after all, my first test was not long enough to dry the filament completely.
I have to be more careful and not leave the PLA spool sitting on the printer overnight. .

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Prusa with cast PU parts, RAMPS 1.3, Wades extruder, J-Head hot end