I have a similar problem with my MSI 5500 XT 8gb. It is randomly crashing on idle doing nothing or watching youtube, office work etc. During gameplay the card is stable, I can play or stress test the card as long as I want. But when the card is doing nothing or almost nothing, it crashes randomly. When it does, sometimes the PC will not freeze, I can shut it down with the keyboard and sometimes it freezes completely. Very annoying problem, it is hard to reproduce and it always happens when you least expect it. There will be something with the card power management, or a bug in the driver, or both.