Using Radeon HD 5770 graphics card. Sony Bravia 40" plugged in as a dual monitor. When watching video the computer will crash/reboot without warning. No error message, nothing. Just black screen and it's booting again. It's definitely something to do with the card and the TV. Computer doesn't crash when just the monitor's plugged it, does crash when there's no monitor, but the TV is plugged in. Almost certain it's to do with video, because that's the only thing that seems to crash it. MAYBE only with 720p video, but I've had it happen a time or two when it's just a youtube video playing (not even on the Bravia).
I've tried all sorts of combinations of uninstalling/reinstalling the drivers. No dice. What precious few scraps I can dig up in Eventviewer make it look like there's some sort of ATI error (duh) but googling the error doesn't seem to get me anywhere. If I google strings like "radeon bravia crash" I find people with similar problems and the end result is always "Give up, buy Nvidia, problem solved" which is great, but not much of an option for me. However, all the posts I've found on it are nearly half a year old, so maybe there's been a fix in that time.
One time during this setup, many Windows installs ago, my only problem was that sometimes the monitor wouldn't wake up after it went to sleep. I'm not sure if this is related. I thought it had something to do with using Displayport. I kind of miss that problem. At least then it wouldn't crash two minutes into a video.