If the maker of the new TV can't fix it, return it as defective. Such things can happen and returning a defective unit is far cheaper than endless phone calls.
-> I did forget one thing. Is the TV software up to date? (often called firmware.)
While I ask, my engineer side of me thinks they deserve returns if they ship product with known buggy code.
It's a simple set-up: digital RCN cable box, old-ish Panasonic DVD/VCR that has been working fine, new Viera. TV works fine when I'm just watching from the cable box, but when I play a DVD the TV keep snapping off after a few minutes and won't turn on unless I unplug and replug the set. Sleep timer is NOT on and set works fine otherwise.