Windows media center playback is very choppy

  When I start my computer and play something in Windows media center, it play
fine but then after a couple minutes, the video starts getting choppy, and
then it just gets worse minute by minute till it's unbearable to watch, but
the audio plays fine.  This happens with both TV and DVD viewing.
Sometimes, the video won't play at all but the sound will be fine.  But when
I play the same DVD in Windows media player, it plays perfect the whole
time, never have any problems.  Anybody know what the problem could be?
Here's my system

Intel Core 2 Duo processor
2gig RAM
NVIDIA GeForce 7950 GT video card, dual DVI outputs and running 2 monitors
Hauppauge WinTV HVR-1800 TV card
Windows Vista Home 32-bit, newly installed with all the latest updates

Also, I had this same system when running Windows XP and never had this
problem.  I'm also not running anything that will slow down the video.

Thanks...

 By all updates to Vista does that include SP2?
What resolution are you running on each of the two monitors?
How much memory on the 7950GT card?
Have you tried with just one monitor?

 
No, I didn't even know about SP2.  I installed Vista on a new drive last
month and then immediately ran the latest windows update.  It installed SP1.

I have two monitors running at 1920x1080, and my graphics card has 512mb.

That's a good idea, I'll try shutting one monitor off and see what happens.

 

Ok, I think we figured it out.  I went back to 1280x1024.  No more choppy
video.  Seems as though Windows Media Center can't handle 1920x1080 very
well.  Don't think it's my video card because everything else runs fine at
that resolution.