I had a problem where I was driving an external optical sensor (LED-photodiode combo) with the same Power supply that drove my uC. The capacitors were connected across the power supply (and not the uC only) and I think the capacitance values I used may have been incorrect, but this problem cropped up: Frequently when I plugged in the wire that drove the sensor, the uC would hang. It must have been the sudden drop in Vcc that caused it.. but by then it was post production, and it was a minor problem for a one-time project, so we dropped it :-))

But I guess this may be the problem...if the relays suddenly load the (common) power supply, the uC hangs...

kundi