I've connected my PC to a HDTV via a VGA lead. It started off fine but after an hour there were flickers on the TV screen but not the PC. 5 mins after that there was constant flickering so you could barely see what was being played.
Check for ground loop. Also try changing the frequency to 59Hz or 60Hz and not more. If you PC or video card is overclocked, try reducing it. Get a voltage meter and the the ground of the PC on one lead and one on the TV; voltage reading should be lower than 20V AC if any. Anything high may damage both units.