Chewing a notch with a MK7 might indicate the plunger is worn and or the wrong number of washers. A new plunger and washers (the only wear part in the system) is about $7 (actually $6.25) from makerbot http://store.makerbot.com/mk7-delrin-plunger.html .
That said, there are quite a few printable spring loaded designs on thingiverse for filament feeders that use a ball bearing roller and eliminate the problem in a very permanent way.
I'm saying this at least part of the "cause" because the motor should not be able to chew the filament and would skip steps rather than the filament giving way if it was pressed hard enough into the gear. Some early Mk7s used rubber o-rings which may not give the same pressure, where the new versions ship with a combination of nylon and metal washers with much less give.
For example. My Mk6 with a much bigger motor will not strip the filament because I can get more than enough tension to skip steps and that's 3mm filament with a ton more torque and force involved.
The flipside we must ask ourselves is why does the pressure or force to extrude increase after several minutes of printing? Via monitoring the temperature readout in replicator G (preferences, show temp during build) in the upper right hand corner, can you verify the temp of the extruder is not dropping below the set temp? If the extruder is not cooling off, the only other option is the nozzle has some dirt or contaminate that gets forces into the taper of the nozzle and jams it. Being too that Makerbot changed geomtery in the new MK8 nozzle that screws into the MK7, you might as well get one anyway. http://store.makerbot.com/makerbotr-mk7-4mm-nozzle-393.html Yes, it sucks that it's $20 but how much frustration is it worth to not have a spare to try?
A final option is toothed pinchwheel and fiament path alignment. It must be exact. If off, then it would effect how much pressure the filament is pushed into the wheel with.
Hopefully, those tips get you to the problem area and it may be all 3.
