[Qbranch]

Uh... can you try making up your own power supply on board? Maybe as simple as just a low-value resistor and a capacitor to filter out high/low frequency noise... maybe wrap your incomming power wire around a ferrite to cut ultra-high-frequency harmonics.

Strange problem... but i sympathize. I had a motion control board I built this summer that worked ONLY when the scope was connected. So... I feel you.

-q