With what driver? I never ever had that once with my old Sapphire HD4870 1GB. Clocked too high it simply locked up, sometimes restoring itself, sometimes it locked up the entire system and i had to reboot manually.

With these 2x 5770's i even get the grey sreens when i oc the cpu too high, screen goes grey, system locks and reboots soon after.

I miss my trusty old 4870 that did 870/1170.