Jittery Premium HD Channels in 7MC... Help!

I've been having this problem with my premium HD channels (HBO, Cinemax, Showtime) since installing Windows 7. These channels are choppy and jittery, they will play fine for a few seconds and then drop frames. The audio is fine, and all my other HD channels are fine, as well.
I've been experimenting with lots of different things lately and I just updated my Hauppauge WinTV-HVR-1800 drivers. The package came with WinTV 7, and now I'm really flummoxed, because all of these channels look great in WinTV but they still are jittery and jerky in 7MC. This must be a problem with 7MC, it either doesn't like my tuner card, my video card (ATI Radeon HD3650), or something else.
Given that WinTV works just fine, can anyone hazard a guess about why these certain channels are so jerky in 7MC?

Have you purchased and installed the MCE version? If it came with a remote and dongle, you did, if not, you should have purchased the MCE version to use MC - or use WinTV, which requires often lengthy customization. As I have purchsed and use the MCE version of the 1800, (which works great) I cannot explain what that customization is exactly in your case.

Hauppauge Computer Works : WinTV for Media Center

I did in fact purchase the MCE edition... although I'm not sure what that has to do with my current problem. Vista Media Center worked just fine with my Win-TV card and I never installed the WinTV software. Now I have to use WinTV just to watch my HD movie channels. There's something wrong with the way my card is communicating with Windows 7. I'm thinking it may be DRM-related, as the channels that I'm having problems with are all pay channels.

Sounds puzzling.

Are you using an HD-STB? What type of output (cable) are you using from the HD-STB to your PC?

Have you tried the new Windows 7 Troubleshooter?

I would:

Download and run HCW Clear from here (lower left):

http://www.hauppauge.com/site/support/support.html

Win TV is designed for use on Non-WMC systems. By installing the WinTV v7 application, you also have installed specific drivers and codec's for use with your tuner which may (will) affect playback in WMC.

Afterwards, reboot, then install (or verify) the latest versions drivers for your TV Tuner, Video Card and Sound (from the manufacturers).

http://support.asus.com/download/download.aspx?SLanguage=en-us&product=1&model=P5E-VM%20HDMI&type=map&f_type=12

Make sure the onboard video is disabled and any apps designed for it (Intel Graphics accelerator) are not running.

I would also verify that you have the required and specific chipset drivers for the Asus P5E-VM HDMI installed (first).

You may also want to upgrade your RAM, 2GB is low for video processing.

The Q9550 is a great CPU, but I am not sure your current mobo really benefits from it as it came out years later.

Additionally, I would contact ASUS support and ask why the Asus P5E-VM HDMI is not on their list of Windows 7 supported mobo's. Others in this series are, but not yours.

http://event.asus.com/mb/Windows_7/
which means this board is not WHQL for W7 and I assume that is for a reason(s).

Can you use another monitor to test if your video plays better with it?

And last, are you using the HDMI output to the Sony and are the cable and connections all A OK?

Make sure you re-run TV setup in WMC each time you make a system change. (not the tuner setup - the TV setup)

If I had to guess, I would look at changing out the Radeon HD 3650 to a high end nVidia video card (or two even as your mobo supports SLI), or add another HD350 at least and dedicate one to your Sony.
Out of curiosity, what are your scores in the Window Experience Index?