Extruder calibration attempts to overextrude  

Extruder calibration attempts to overextrude

So I've got a pretty much functional 3d printer now. It can print some things just fine, I've set the first layer height well, but it still has a few issues printing. After some research, I discovered extruder calibration. I decided to give it a try, and immediately ran into some new and exciting problems.

The process I'm trying to do is as follows:

1) Preheat the extruder and load filament.
2) Mark the filament 150mm above the point of entry the extruder
3) Send gcode to printer to print 100mm
4) Measure difference from expected print amount and actual print amount.
5) Make offset.
6) Repeat until good.

Unfortunately, everything goes haywire on step 3. When I send the gcode to the printer (G1 E100 F100), it'll immediately try to spit out 100mm of filament in under a second. Using the pronterface extrude function, setting it to 100mm extrude at 100mm/min, the same thing happens.

Would anyone have any ideas what might be causing this? My first thoughts were the bearings being too tight (or loose), but they seem to do a decent job of gripping the filament. When removing filament, the filament is slightly marked from them.