Safe to "lift ground" on projector? (1 Viewer)

After hooking up my new Panasonic XP30 DVD player to both my 53" RPTV and my Z1, I got horizontal rolling lines. They went away after unhooking the XP30 from the RPTV, but I want to be able to use it with either my projector and TV. I currently "lift the ground" on my subwoofer's three prong plug using a "cheater plug" to alleviate a ground "hum". It occurred to me to try it on the projector since it is plugged into an outlet in the attic and the rest of my HT gear is plugged up in the living room (I thus probably have a difference in ground 'potential'). I put a cheater plug on the Z1's plug and it worked! The XP30 is hooked up to both RPTV and Z1 and I have no rolling lines! Now, is it safe to leave it that way? Will there be any negative affects? Can it negatively affect picture quality?

thanks!
Jim

Side question - Is there something in your house that is causing noise on your a/c lines?

Per a suggestion, I am going to try to find a ground loop isolator for my TV cable feed to see if that helps.