Stepper BBQ-Mine's well done.

This weekend I was using my CNC plasma gantry when all of the sudden
the y axis quit doing what it was supposed to do.
I have used it several times but never for more than 30 minutes at a
time. Anyway, this time I had it on for a couple of hours and it just
overheated the motor and I am guessing it shorted out internally. I
measured the coil resistances and they were all messed up. And there
was the funny smell of melted plastic (Hmm?)
I was under the impression that steppers were pretty impervious to
heat and overcurrent. How can I prevent this in the future?
The motors were SLO-SYN 180 step/rev. (34's, and pretty old). The
driver is an Anaheim unipolar with a little pot to set the current from
about .5 to 5 amp. I guess I was running the pot about "half way up".
Advice on setting current levels or monitoring the temp? Thermal
shutoff?
I don't have the cash to keep replacing motors from my Ig-no-rance.

Thanks