[Alan Anderson]

Just a shot in the dark -- are your mecanum wheels mounted properly? If you have them in the wrong orientation, they will be fighting each other when you try to rotate the robot. The rollers should form an "X" pattern when you look down on them from above, and should make an "O" or diamond against the carpet.

The other thing I'd look at is whether you have each motor going to a separate speed controller. I have seen very strange power symptoms when two motors are accidentally cross-wired between two Victors.