I don't think it is related to Win 7. I think it is related to the Monitor and or video card and or the cable you are using. If you have 2 monitors that are the same make and model just use the one that works all the time on the dual monitor win 7 setup.
Windows 7 doesn't just disconnect one monitor at random. And if it is always that same monitor then it is a monitor, cable, monitor connection port or video card and or connection port on the video card Problem.