the screen may be getting an earth thru the video cable shield wire.
try running a new earth wire for the screen, try an earth point close to it first since that easier, then if that still dosnt work run a wire to the earth input terminal on the amp.
since that may be easier to get to in theory than your normal earth point and for testing it should suffice.
of course you could just get a power noise suppressor/filter and put it on the screen.
those small black boxes you often get on car stereo's power wires.
if that dosnt work you could try putting a ground lift box on your RCA cables to the amp