Radeon 5670 strange issue

I recently upgraded my PSU and GPU. I don't remember the exact wattage but something like 450 and a Radeon HD 5670. Now while I'm playing games, notably SWTOR and TF2, the game will freeze for a few seconds, then lots of small multicolored dots show for quarter of a second, then things go black. In SWTOR I hit a mini-loading screen and then I can play again, but it becomes an issue while I'm raiding, especially if it decides it wants to happen every 15 minutes. In TF2 when things come back I am still playing, but the entire screen is black. I can move, shoot, open menus, everything, but it is all black so I can't see a single thing. I've tried updating drivers and I'll try again but does anyone know if there's more of an issue going on?
PSU: 450W
GPU: ATI Radeon 5670 1GB DDR(3 or 4)
Processor: AMD Athlon II x4 @ 3.00 Ghz
RAM: 6GB DDR3
OS: Windows 7 64 bit
That's what I know off the top of my head, if you need more just ask. Thanks.

Also, the old GPU was a NVIDIA 420GT or something and it never did this.