MakerBot Industries

The idler wheel could be out of round and if it isn't seated level on the bearing or your filament is out of round too, that might exaggerate the problem. Still, I wouldn't expect it to be that sensitive to pressure. The idler is laser cut, so it should have fairly tight tolerances and if you adjust with the rod, that gives the teeth some serious bite on the filament.

Does the filament ever slide off the pinch wheel? Have you cleaned the teeth on the pulley?

If you check that and it doesn't help, then you might try rotating the pulley so that when you put the rod in, it rests on two teeth instead of just one. Then you should be able to increase the compression a bit without accidentally over doing it. There isn't anything to really lock the idler in place either. It may simply work itself loose over time. I doubt that metal-on-acrylic provides enough friction to stop it from creeping down the slot.

EDIT:
Another thought I had was that maybe it is getting stuck in the heater barrel somehow. I remember that the wiki said something about having the barrel tightened too hard against the barrel insulator. If it is to tight, then it pinches on the filament or something like that.

everything dazeddnc said could be right. just another thought: it could be that your temps in your gcode are too low, so as the temp drops, the abs doesn't melt as well, and you get stripping (Which is common if your temps are too low)