Some things I would try...
Make sure the deck has a ground strap and it's attached to a solid grounding point.
Sand down all your grounding points to insure a good metal-to-metal contact.
Check all other grounding cables especially the amp ground.
Take a spare set of RCA cables, run them direct from the deck to the amp.
You could always beef up you battery ground and alternator ground. This is what I plan to do with mine.
Good Luck
Seth