Grainy TV reception

Hi,

I've just purchased an HTPC which is connected to a 40 inch Samsung HD tv via an HDMI but the TV playback/reception is very grainy. The image via the TV tuner is pefect but via the HTPC it is grainy, the ariel signal is boosted close to the ariel and the split close to the TV/HTPC and regardless of which connections I use the quality of the picture through the HTPC is poor.

The HTPC is running at 1920*1080, DVD playback is excellent, and recorded TV play back is not bad (not perfect) and if I play the TV through a smaller window the quality of the picture is better.

The HTPC consists of :

Gigabyte GA-MA78GM-S2H motherboard with ATI Radeon HD 3200 Graphics
Hauppage Nova T-500 Freeview Dual Tuner
4GB Memory
Zalman HD160XT+ Case

Where do you think the issue lies ? In the Graphics card, the TV Tunder Card or within the drivers.

Any help much appreciated

Thanks

Mark