Filament stepper stalls during load, then overheats

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Hi all- I am running into an issue with an Ultimaker II.

When inserting filament, it loads properly for a few seconds. When it speeds up to load quickly, it just hangs with a really awful noise (not moving), then continues another second after it has slowed down. As if the stepper started moving too fast and couldn't handle it. After the motor goes back to moving slowly, it ends up moving through fine and extruding.

However, the motor overheats, and ends up melting the filament where it touches the knurled knob, causing all prints over an hour to fail.

I've tried a replacement stepper, but the issue persists.

Video of the issue here: http://vid.me/Hdbu

Thoughts?