Sounds like it may be a grounding issue. First thing to look at would be the ground wire on the power cable, make
sure it is tightly attached to the chassis, next thing to look at would be all the wires connected to ground, it's probably star grounded, anyway
make sure all the ground wires are snug.
Oh yeah, don't forget to unplug it and bleed the voltage off the caps, otherwise it could be a shocking experience, it hurts.