What exactly happens? It starts out fine, but after (several) hours it slowly slows down or does it suddenly drop a lot in speed? How does the video play exactly? Good sounds with stuttering video or even delayed video?
I've seen similar issues on my HD4400 IGP, where my IGP simply couldn't keep up with the hardware based video decoding, and switched to software in stead. I haven't been able to figure out yet why it happened, though running my IGP at a slightly higher speed helped. Is there a chance your computer switches from your GT to IGP by chance?