I've been happily mining away on my 5830 for over a month and then a few days ago it became unstable. After a couple or three hours the computer crashes and reboots. I haven't been around the computer when this happens, but when I get back the computer is no longer mining and is asking for the password to get back into my desktop. At first I thought it had something to do with the latest round of MS updates (Win7) so I did a rollback of my system, but that didn't help. I upgraded poclbm to the latest version and that didn't help. I use GUIminer.
I got a program called BlueScreeView to determine what was causing the crashes and it says it's the "ATI Radeon Kernel Mode Driver"
I'm running out of things to try, suggestions appreciated.
Agreed with Ates, my suggestion would be to try turning down the clocks on your 5830, when mining see if that helps. Can also run GPU stress tests (like 3dmark) to see 1) if it crashes and/or 2) if you get video errors to help see if the card aging and abuse has finally gotten to it.