Computer shutting down during gameplay.

I had some serious RAM issues last year and tried endlessly to solve it with everyones help on this forum, with no luck I just ended up purchasing new RAM and a new PSU and the problem seemed to dissapear. Recently I have been experiencing instant shutdowns while I am playing higher-end video games (Crysis 2, Black Ops, Dragon Age 2). Usually I can play anywhere between 10-30 mins until my computer just shutdowns, no bluescreens or freezes, just an instant shutdown. I'm sure it's not an overheating issue because my gpu/cpu temperatures remain constant and in safe variables, also my rig door is off for better cooling and this problem has only starting occuring recently. I'm also doubting it's a PSU issue because i'm using a pretty decent 600W gaming PSU which is near brand-new and meets my GPU requirements.

I've tried registry and spyware scans but the problem still occurs.
Any help would be greatly appreciated because I've had heaps of problems with this rig from the whole faulty RAM situation and now this instant shutdown stuff (my bank account would appreciate it as-well).

Specs:
CPU: AMD Phenom X2 550 Black Edition (Unlocked - AMD Phenom II X4 B50)
RAM: 4GB (2 Sticks) Patriot Sector 5 DDR3 PC-12800 (Clocked to 1333 because 1600 was unstable)
GPU: VisionTek Radeon HD 5670 1GB GDDR5
MOBO: Gigabyte GA-MA785GMT-UD2H
PSU: OCZ ModXStream-Pro 600W PSU
Monitor: Dell ST2010 20" HD Monitor



I've tried tuning my Windows visual settings to flat performance, helped the frame-rate of my games but the problem would still occur.

Moved computer over by the window to see if it's a heat issue, not sure yet.

check the fans make sure that they are circulating the air flow correctly

I went into the Catalyst Control Center and set the manual fan speed to 100%, the automatic setting was around 30% during gameplay and I haven't had a problem so far. I do have a fan at the bottom of my case that is blowing air directly to my videocard and I'm wondering if I should flip that fan upside down since the GPU fan is blowing in the direction the case fan is blowing in.

that may help pull the heat from the gpu