With my HDTV I've seen flashing in these 2 cases:
1) I had a BlueKangaroos 28AWG 75ft connected to the HDMI port of my PC - this flashed really bad.
a) I had no amplifier at the time. I ordered a 24AWG with a built in repeater, and they sent that junk. I promptly sent it back when it didn't work.
b) this is when I made the mstake of going the route of VGA. after all my 1080p monitor was VGA, and the HDTV also had VGA-in.
2) I had a Eforcity VGA to HDMI converter connected with a Short cable or long cable.
a) This worked differently then my VGA current converter. This had two settings, and the specs are different. In 720 mode it would work. But in 1080p, it would Flash worse than it does now. Even if I connected it from my laptop at close range. So I assumed that the Eforcity converter was bad - and I sent it back...
Is the TV bad---? that is one possibility. It connects HDMI to the cable company and receives a brilliant 1080i picture. But I've never tried to connect to a 1080p source, and it yet work without flashing. I do not have a blueray player to test.
No one thinks I should try the extension cord test?