Your limited testing is flawed. If the VDDC's overheat they will shut down the card to protect it. If you dont want to have a crash when testing artificial means such as this stability benchmark then increase the fan speed to cool the VDDC's. Obviously underclocking acomplishes a similar goal as they dont get as hot. No games are affected just artificial stability tests.
That is a wonderful response. I was considering a 4890, but now I'm not so sure. Does XFX make one with the 4 phase VDDCs?