TV in Vista Media Center VERY jumpy - please help

I need to be able to watch TV on my PC because I'm selling my old rear projection Mits and it will be awhile before I'll be able to buy a new LCD TV. I have Dish network, and I know I cannot use the full fucntionality of VMC to controll the set-top box (guide and DVR and such), but I want to just use the PC as a display basically and continue to use the set-top box as the controller.

Here's the problem: the picture is so jumpy it is pretty much unwatchable. I have my set-top box connected via s-video and for audio, a RCA to 1/8" stereo plug. I have media center set to channel 3 to just receive whatever signal the set-top box puts out. The picture quality is fine and the audio syncs up okay, but it's constantly "skipping".

My hardware specs are as follows:

Core 2 Quad Q8300
6 GB DDR3 RAM
Nvidia 9800 GTX+ w/ 1GB RAM
Hauppauge WinTV-HVR-1250 TV card
Vista 64-bit
24" widescreen (Dell) with VGA, DVI, and HDMI Input (connected with DVI)
1920x1080 res.