My power supply is the 12 and 5 volt rails and ground from a computer power supply, I'm using 12 volts because I have puny motors. I get a steady voltage from it whether its connected to the board or not. Is your voltage fluctuation occuring only when you activate the motors or is it any time it is connected? If it is only when the motors are activated I'd bet that you have you motors connected wrong, if its all the time it may be a faulty board. Something to try would be switching your + and - inputs, they might be wrong
