Hi

If you have rate dependent thermal motor shutdown, the answer may be to drop the speed. If it's purely thermal, it should happen when the printer has been on for a while. In that case, you may have to much current headed into the motors. The little paste on heatsinks do very little for the drivers. You can get a little more benefit from a fan. Even there the benefit is less than you might think. The way the chips cool and the pc board layouts don't generally work very well together.

Bob