What OS are you using?
I've been on top of MS and ATI for months to correct an issue with 1080i ATSC playback on Vista with the 690g chipset. It manifests as a green screen using the Microsoft MPEG2 decoder, and stuttering playback with other decoders. It's fixed in Vista SP1, which is available for public testing now. This fix is not available via driver updates.
May or may not be related to your issue, since I see the problem with Media Center. But if you're on Vista, it may be worth trying.