Are you running cgminer? Even weak cards would pick up again and you would notice HW: value increase from 0 for every lockup or other failures in the hardware.
I am running cgminer 2.4.2 on Xubuntu 12.4 (Catalyst 11.11 drivers with the included 2.5 SDK) on my rigs.
Unfortunately these locks are hard locks where one of two things happen:
1. I can log in under a root SSH session and see the cgminer PID is zombied to point even a Kill - Sig 9 will not kill it. Most of the time I can do a reboot command and gracefully reboot.
2. Cannot get a SSH session and have to hard reset, power-off & on.
Both rigs are using different MB and CPU and I separated them among two lightly used circuits in the house. I keep power supply demand at least 100 watts under rating.
For testing, I have reduced both rigs to 2 GPU and I have already found 1 card that locked up under this lighter condition.
Wow ok I guess even cgminer cant reset cards that are just purely into self destruction.
Yeh seems you need to call the GPUlance to do some CPR on those cards.