Choppy Audio/Video Using GT430 Full-screen Over HDMI?

i've just built a htpc using some old bits from a gaming pc, but when streaming videos full screen the audio and video are very choppy, with the audio including crackling sounds. for example, if i watch a 1080p video on Internet, it works reasonably well (with the odd pop from the audio) but as the controls overlay (with the play, pause and video slider) slides away the video slows down and there is a lot of crackling from the audio. there is an even worse problem with bbc iplayer and skygo when watching on fullscreen as the entire time the video almost seems slowed down and the audio is very choppy. if however i watch a downloaded video fullscreen, there are absolutely no problems at all.i have only just build this pc so i believe all drivers are up to date. i'm using windows 7 and have therefore disabled aero to see if that helps (it helped slightly). the hardware is as follows:

cpu: core 2 duo e8400 at 3ghz
graphics card: nvidia gt 430
motherboard: gigabyte ga x48 ds4
ram: 4gb ocz reaperx

for my old gaming pc i used all the same parts apart from using a gtx 460 for the graphics. this was then set up exactly the same way as this new htpc (into a samsung tv via hdmi from the graphics card) and yet worked absolutely fine. so the only thing that has changed is the graphics card, but the gt 430 is supposed to be able to easily handle hd video and audio.