I have the same problem quite regularly, on a 5770. Often the display driver just throws up and error which is ok because I dont need a restart but other times it will freeze and I need to reboot to bring it out of the freeze.
I'm using guiminer, also. Is there any way to search for corrupt drivers?
I overclocked with MSI Afterburner and speeds went up over 200 (unstable tho when it it 99-100% gpu usage). Now I just underclock the memory and it goes to nearly the same speed. Not sure why it has been so unstable, I was thinking of reinstalling drivers and afterburner to see if it helped. Any advice appreciated