This still sounds like it's power related, give me the entire specs of your system. Cpu, power supply, ram, motherboard, hard drives etc.
There is another issue though and maybe you might want to use driver cleaner to fix it up. It sounds like you most likely didn't uninstall your old video drivers properly. You should always boot into safe mode when removing drivers. It'd be nice if the graphics card companies told you this but it's really the best way to uninstall that stuff. So reboot into safe mode, uninstall your drivers and then reboot back into normal windows and re-install the proper drivers for your card.
If it's not your drivers or power supply then there is a problem with the card.
Prepare for termination! It is the only logical thing to do, for I am only loyal to Megatron.
