I've had similar problems on a table top mill, have not yet found the answer.
Your drivers are still listed, and Superior still makes steppers, and list lots of them on their web site. You might want to give them a call..
Their web site does give specific precautions on keeping the working temperature on the drivers down, mentions adding heat sinks and increasing air flow.
You can buy digital readout aquarium thermometers for well under $20. Be interesting to clip one to the heat dissipation area on the drive in question and see what its operating temperature is.