Well somethnig to try is providing some additional hi-frequency bypass capacitors on the power input to the driver board. An 0.1uF/100v in parallel with an 0.01uF/100v should do, this will eliminate any switching spikes from the motors going up into the power supply and upsetting its regulation. Also to absorb back emf a large electrolytic capacitor might be useful, something like a few thousand uF at 100V.

I agree with John tho, if you can try a linear power supply then do so...