Check your crash dump files for clues if you see any advanced micro system or Bc Code : 3b 0X0000003B this error is linked to ATI graphics drivers crossing over and passing bad data to the kernel code which will cause crashing due to corrupt drivers, also it could be idle crashing which sounds likely because you ran some heavy tests and system worked fine at med to heavy load but crashes at low usage, if other ATI info shows up in crash dumps try starting a new power plan and save your current plan,.. changing your power plan to high performance, then click on change plan settings then change advanced power settings, disable all idle, sleep, min power options that shut down system, or hard drive and set min to zero, max to 100, restart and try that,.. you can always return back to your old plan if it does not solve issues, but seems to work for most cases.
Windows OS has been causing some older ATI cards to crash when idle or under vary light load like surfing internet, or streaming video. They know about ATI idle crashing issue,.. check on Windows forums lots of folks having same issues, no patch or update yet. Your card is more than likely fine,.. Good Luck
Edit : To confirm idle crashing, play a game on normal settings for at least one hour to confirm graphics will work at med. to high usage, if your system does fine then close game, and just open a internet browser do not use any thing, let your rig just set at low or idle usage, it should crash within 30 min.
Edited by Braegnok, 18 June 2013 - 03:27 PM.