x2.
run an extra ground as well. just from anywhere real quick to see if this helps. just because you have a good metal to metal contact doesn't mean its a good ground.
you can leave the ground you have now, just run a temporary one and see if it helps. sounds like bad connection somewhere, either ground or power.
also. what size fuse do you have? you might have to small of a fuse on your power wire for your amp. you should have a fuse at least the same amps (fuse rating) of the fuses in your amps.
if your amps have 60 amps worth of fuses then you should have at least that much on your power wire.