Thanks, I'll try dropping the clock speed.
Run it at stock speeds for a while. I would also set intensity to 19. Once it is stable at stock speeds then you can start playing around with bumping things up, although with scrypt mining it's more about ratios than how far you can push everything.
You can also look at recommendations for 5800 series cards from this thread -
https://bitcointalk.org/index.php?topic=117221.0good luck (again).
Thanks. I've dropped clock speed to 850, and RAM back to 1200. I'll run it at 20 intensity and if it fails again, I'll go back to stock speeds for a while.