Preview playback video is freezing but audio continues?

I am playing back video and the video freezes and then jumps a couple of frames. Then play it back again and it does the same thing in a different area, so I know the footage is ok. I have a quadro 4000 8GB ram and my processor is barley moving - any idea where the hang up is - am playing back at 1/2 frame rate. footage is 1920x1080 MP4 - just pre-viewing no effects just viewing.
never notice this before?

PC
I7 2.8 Ghz 8GB Ram
Win 7 Pro 64bit OS
PNY Quadro 4000

3-4 TB HD
Vegas 10 32bit & Vegas 10 64Bit & Adobe Production Premium CS5.5

Camera
Sony EX1 shoot in 1920x1080 30P