I would second that it could be the PSU, may as well re-install the drivers for the GPU (Make sure to tick 'clean install' if you do). May help, may not. But it would be worth upgrading the PSU nonetheless.
EDIT: GPU temps would also be a good idea to post, it could be overheating and throttling for all we know.
Sounds well ill update you whit temps tommorow after work in already off pc