Nvidia 9600GT doesn't playback HD video

Hey guys. Major problems here. I'm racking my brain trying to figure this out and no one seems to have any answers.

A week ago I bought an EVGA 9500GT so I could playback HD nice and smooth on my 24" 1080p monitor. But HD video was nearly unwatchable. Completely sluggish.

I upgraded from XP to Vista 64 hoping that might solve some issues. Nope.

I pulled the video card and put my old 7100GS back in my system. HD video plays back nice (better then it did in XP surprisingly) but not entirely smooth and constant audio sync issues. 100 times smoother then the 9500GT however.

So I figured the 9500GT must be defective. I returned it today and bought a Zotac 9600GT. Same exact problems as the 9500GT.

Here's the thing. Games playback great. I'm not a big gamer but I am now able to play Fable at full 1080p resolution with all details maxed out. SD video files also playback ok, not wonderful but ok.

The only thing that doesn't playback is HD videos of any type.

Here are my system specs and software/file types/codec packs used:

Hardware
Core2Duo 1.82Ghz
2GB DDR2
ECS P4M890T-M2 motherboard
3 SATA hard drives (videos are not on the OS drive)
9600GT
450watt Power Supply

Software:
Vista 64 (fresh install)
Media Player Classic
VLC
Smplayer
WMP11
iTunes 8.1
CCCP (also tried K-Lite codec pack)

Videos I'm trying to playback:
720p MKV files
1080p VC-1 file
720p iTunes HD MP4 store bought videos

All of them playback horrible. I mean literally so slow they are unwatchable.
I have the newest Nvidia drivers installed.

No there is no HDMI involved at all. I'm just trying to playback the HD videos on my 24" desktop monitor in Vista 64.

DVI out to Westinghouse 24" LCD 1920x1200 resolution.

I don't know what an HD advisor is but I'll look it up.

I just bought some episodes of battlestar galactica off of itunes in HD and want to play them on my computer. But They are so sluggish they are unwatchable. I mean the video stops while the audio continues, then the video plays three frames and stops again for several seconds.

It does the same thing with VC-1 files and 720p MKV files. Multiple different players and codec packs have been tried.

Thanks for the feedback guys. I have MPC installed and I followed the guidelines on the guru3d site but I'm still getting choppy playback. Especially with 1080p files.

But I think what gets me even more is that my iTunes HD files playback worse. Hardly at all, when they playback much better with the old 7100GS installed. Why would this be? Obviously it can't be my CPU or RAM because if that were case I should have been getting the same horrible playback with the 7100 card.

I figured it out. I can't believe it, but the problem was, my machine was running too hot. I downloading a CPU temperture monitor and discovered that each of my cores was running at 101 degrees celcius.

After taking the side panel off my box and moving it away from the wall, my machine is holding between 80-90 degrees and the videos are all playing back just fine.