Display driver NVIDIA stopped responding, pls help!

Hi guys,

I'm having a pretty annoying problem since a couple of days, I'm not sure exactly when and how it started but here's what I'm experiencing;

When I don't touch the mouse or keyboard for a little while, say 2-3minutes, and then when I press a key or move the mouse again, the screen will go black for a second, come back up and give me this message: "Display driver NVIDIA Windows Kernel Mode Driver, Version 310.90 stopped responding and has successfully recovered." As long as I keep typing or moving the mouse it doesn't seem to happen.
When I play a game however the framerate will drop heavily after some minutes and then I have to alt-tab, screen goes off and on again when I do, alt-tab back to the game then framerate is fine again. But as you can imagine it's pretty annoying having to alt-tab every couple of minutes.
Now I'm by no means a computer expert (like I hope you guys are   ) but my little brother is pretty good with them but after trying some different possible solutions even he had to throw in the towel. I really hope someone here can help me find a solution.

Here's my pc specs:
i5 2400 Biostar motherboard (not sure what type but I'll find out if you need to know), 8gb Ram, 128gb SSD, 1TB HD, and probably most important; asus geforce 450GTS. System is about 8 months old I think.

What I've tried so far:

- updated nvidia drivers
- try several older nvidia drivers
- set nvidia power managment mode to 'prefer maximum performance/adaptive' (tried both)
-switched off intel speedstepping

I think that's about it, videocard doesn't get too hot, I can't think of anything else but it having something to do with going in somekind of idle state.. I dunno, I'm totally lost on this.
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