Prints randomly skew x or y

Prusa, RAMPS 1.4 on a 2650 r3, Marlin, Slic3r 0.7.0, Pronterface, Windows.

I get one very nice print (in this case a footed vertex) and after clearing the build platform, try to print another with the same gcode and after 4-5 layers the whole print skews 10mm or so towards the x-axis end stop.

-or-

Every time (and I've tried at least a dozen times) I try to print an extruder, at some point the printer forgets where it's been and skews the print either x or y, sometimes right after first layer perimeters/start infilling. It seems like the more complex the part, the more likely or promptly the printer starts skewing the print. I've had mixed results with gears -some perfect, some hosed.

What I've done:

Swapped Pololu shields (I have an extra)
Swapped RAMPS (got a friend's)
Swapped Arduinos (got a friend's)
Tried different host computers
Slowed communication between host/printer to 115k
Rerouted conductors so motor leads and hot end leads aren't near each other
Calibrated per the calibration page (motor controllers/extruder/buildplatform/etc.)
Regenerated/rechecked gcode

I have not swapped motors, yet, but that's next. One motor (extruder) is unipolar, the rest are bipolar, could this cause a problem?

Any help is most appreciated.

Lower your "default acceleration" in Marlin. I had the same issue...mine mainly was on short repeated moves causing skips.

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Lower the initial speed and acceleration in your firmware. Also clean and oil your bars.

Thanks! I'll give that shot tonight after work...

Do you have a fan blowing on your Pololus? Do you have heat sinks on them as well? Overheating of the drivers will sometimes cause skipping. This is the thermal protection that is built in to the boards.

The acceleration setting is more likely, if you have adequate cooling.

What surface are you printing on? If glass are you cleaning it as you need to with something like isopropyl or I use methylated spirits otherwise my prints (PLA) don't stick properly

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Experimenting in 3D in New Zealand

Well, chalk this up to a violation of the KISS principle.

The issue was the x-axis pulley not being properly assembled (wrong nut) and so the pulley occasionally free wheeled. Doh!

Thanks for all the help and now I'll be starting another thread...