I know this is really old... but I am having this exact issue right now and wanted to share my observation. As soon as this started to happen, I cleaned the dust out of my computer. I just got this computer in May of 2010 and the issue started in Dec 2010. I have a GeForce GTX 260 video card and Thermaltake TR2 RX 750w power supply, so it should be able to watch Instant View pretty smoothly I'd think.
When I first started having this issue, we suspected it was the power supply or even my BIL that works at Microsoft said my cat was touching the power button on the back of my computer -- yeah right -- guess who gets blamed for all Microsoft issues around here! My computer literally just shuts off, no warning. I can turn it back on and it acts like nothing happened, just starts right up. All tests and BurnInTest runs smooth and no problems are reported.
I had recently started watching tv shows on the Instant Viewer, so I watched a crazy amount of hours on Instant View. I thought it could be a software issue (since I really don't want to deal with replacing my power supply) since it just started happening randomly and that was the only new thing I was doing on my computer. I stopped using Netflix Instant View for a few months for tv shows only. I still watched movies without issue. My computer did indeed stop turning off randomly.
Last week I just started watching TV again, forgetting my issue a few months ago because it disappeared. And again, guess what started to happen? My computer turned itself off randomly! It did it for 2-3 days in a row! After the 3rd day, instead of hitting refresh when I bring it out of sleep mode, I hit the back button and rehit the play button from the view movie page. The issue went away completely. Have not had the computer shut off once.
My conclusion was, since I watched only movies before, I never really put my computer to sleep with the Instant Viewer saved in my browser. But when I started to watch tv shows, I almost always put it to sleep in the browser because I would come back and keep watching the next episode. After 5-10 minutes after hitting refresh in the Instant Viewer, my computer would restart. To prevent it, you have to exit the Instant Viewer and come back.
Coincidence? I have no idea, but that is my experience.