extruder motor gets too hot

I have a v-minion with a vz-hextrudort and an ebb 42. It is running the moons 8tooth motor that is recommended. The stepper motor eventually gets to hot causing the PLA to soften and eventually jam. I have tried a different motor but it still gets too hot. What are some things that could cause the extruder motor to heat up like this. Thanks for any help.
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