If it's not moving until things heat up, then I would suspect the drive. Possibly the motor. Surely electrical though. I personally have been through the same things and found each time that it was mechanical. I have now locked all my connections with lock tighted set screws and I drilled seats for them as well. Basically now like a keyed connection. Keyed connections can still slip, but it sounds like the motor is simply loosing power or torque.
The spring thing may help. You should be able to rig up something temporary just to see.
If that helps, I would look into the drives, motors or PS.
__________________
Lee |