Ive had similar problems , did you build this sytem ? There are a few things you can check to see what helps .1st off make sure you have no memory conflics use this tool MEMTest 2.1
. Run some extensive tests on your memory at boot up .I usually run it all night if no errors occur for an hour or so . Check your AGP voltage setting i run both my ATI 9700 Pro and the 9800 Pro at 1.5 v..... next This may make a differnce Boot to bios look in Advanced /Chip Configuration.
In the line Video Memory Cache Mode see if your Configuration Option is UC or USWC .
USWC(uncacheable , speculative write combining )
is a new technology for the memory video of the processor.It can greatly improve the display speed by caching the display data.You must set this to UC (uncacheable) if your display card does not support this feature . Sometimes conflics can occur with these 2 settings .I set mine at USWC this may be of interest .
I use Driver Cleaner 2 to remove any Drivers that may still linger around after uninstall .You may want to check if turning off Fast Writes in the ATI control Panel makes a difference . Use a 3Dmark tool to make sure your system is running stable i like 3DMark 02 se for general tests . "Finally " I own 2 ATI cards and they tend to run hot , but usually the system just locks up but rebooting can occur as well . Make sure you have good ventulation around the card .Ive acctually modded both my ATI cards with bigger fans to fix heat issues . Hope this helps in some way