iTunes HD Playback is Unwatchable

I don't get it. I have a pretty decent machine. Specs listed below.
I bought a couple of HD episodes of Battlestar Galactica off of itunes and trying to play them back they are really jerky and pretty much unwatchable. The quality also looks horrible. I'm talking the colors and resolution. I don't get what the problem is.

My other HD (720p and even 1080p)content that isn't bought off of iTunes plays back just fine. It looks great, ultra smooth even during fast action scenes.

Now I just updated my machine yesterday. Added more ram and the 9500 GT card hoping to get HD to playback smoothly. It worked for my other HD MKV files, just not the itunes stuff. The BSG itunes episodes I bought playback worse now then they did before I upgraded the ram and GPU.

Why would the quality and playback get worse with the itunes stuff and better for the other HD stuff after the upgrade?


Upgraded Machine Specs (BYO)
Core2Duo 1.83Ghz
2GB DDR2 (old machine spec: 1GB DDR2)
EVGA 9500GT 1GBDDR2 (old machine spec: 7100GS 128MB)
Windows XP Sp2
24" 1920x1200 DVI connected LCD.
iTunes 8.1
500GB SATA HDD

 

Not even a decent guess from anyone? It's driving me crazy.