Printer offsets print

Trying to calibrate my Prusa i3, running the Marlin firmware with a Pronterface/Slic3r combo. For some reason on many of my prints, the printer decides to offset the print partway through. E.g. the 50mm tower prints fine until at some point it moves about half the box's width in the x direction and continues printing normally. That's about 5 mm so I don't think skipping steps is possible. Opening the box in slic3r's Plater shows a perfectly straight tower. It's done so on a couple of other prints too, including the 20mm box. I haven't tested too extensively since each print takes a while and I don't want to keep wasting platic.

Any ideas on what could be causing that? My only thought is that some of my Polulu stepper driver heatsinks are getting rather hot to the touch, and I don't have active cooling in place. Could the X driver be overheating and stopping at some point, causing the offset?
Thanks in advance!

The belt could be slipping. I'm going to throw the usual suggestion out there with any offset issues: check your belt tension. Also check to make sure your pulley grub screws are tight and the belt meshes well with them.

Quote
anvoice
My only thought is that some of my Polulu stepper driver heatsinks are getting rather hot to the touch, and I don't have active cooling in place. Could the X driver be overheating and stopping at some point, causing the offset?

Measuring volts from the pots on the stepper drivers to any ground with a multimeter (I use the outside part of the USB plugged into the board) gives you a number that represents the amperage sent to the motors during operation (I believe 0.1 volts is 1 amp, not sure on this). You should be able to run them up to 0.4 without any active cooling assuming no enclosure. If they're higher than that, try turning it down and see if that helps.