I'll try that - the head was already on a chair away from the cab, so I don't think it's vibration going back into the head.
I'm going to have it checked out.
I've been playing with my old band recently and have been borrowing a friends SVT3 and ampeg 4x10 and 1x15 cabs to practice with. I'm getting my own cabinet redone but I've gotten this new Mesa Buster head . I played for about two practices with the SVT and everything worked fine. The next time...