Are there some 3rd-party power management softwares running? Better uninstall them and just use system default (high performance profile)
Or you can update your BIOS. A new version may help.
Yeah i've recently bought a AMD 7850 to update my 9600gso...i used driver sweeper too. What i suggest is: clean drivers in Windows Safe Mode.
common knowledge, windows loads minimum drivers in safe mode, without graphic card driver. try it.