Okay, installing new nvidia drivers mostly fixed the problem.
I ran a test with TV3 and by the stats it seems to be okay (55000 frames rendered, 9 dropped, 5-20jitter). I didn't watch it the whole time so i dont know what it looked like.

However now watching TV1 with the news, im seeing small glitches every now and then, which aren't contributing to dropped frames, though i think it makes the jitter go up. Stats at the moment are 104000 rendered, 26 dropped, 14 jitter +-5ish. There is about 2 layers of aluminium foil between the graphics card and the TV tuner (8500GT and HVR-2200) to try and stop RF interference.
I got this with the Olympics coverage too.

Guesses as to what is causing it?