One of these stepper motors that I have is not working properly:

http://www.slidesandballscrews.com/s...ml?cPath=45_81

The max rpm has suddenly decreased greatly - won't go above 620 rpm with no load. It just misses steps, or just stalls whilst accelerating.

I've tried it on to other identical, PM752, drivers running on 70v and get the same symptoms - so it's not the driver that's the problem.

My other 3 of these steppers are all fine...

Any ideas what could be wrong / where to go from here?