Artefacts, mouse and keyboard going off under load and weird behaviour

Hello guys.

I shall kindly ask for your help as so far any method of troubleshooting has failed me.

So whenever I enter a game, after a few seconds I start getting visual artifacts (black lines, etc). That might be ocasionally followed by general audio stuttering, as well as mouse and keyboard going completely undetected anymore by the system, even though, after this happens, I can still hear the sound playing in the background, apparently the system is doing fine.

My system also freezes pretty much completely when I use the Prime95 FFT test. I know Prime95 puts a lot of stress on the system, but complete hang up should not happen as far as I know?

Another thing I have noticed is download speeds are sometimes extremely slow (definitely slower than usual), even lagging on YT stream and such.

Event Viewer isn't helping me isolate the issue either.

PC was running fine and smooth until this started happening. Didn't change anything in it's configuration hardware or software (until this happened and I decided to format).

So far I:

- Reformatted my PC (running Windows 7).
- Made sure all my drivers are up to date.
- Tested my RAM with memtest x86 , turned up clean, 0 errors.
- Cleaned my PC of dust >_>.
- Made sure temperatures are in range.
- Set back CPU to default frequency (OCd it to 3.0 from 2.5 GHz / core, but this happened one or two years ago, everything went fine until now)
- Tried underclocking my GPU
- Tested with Prime95 FFT Test, PC hung up.
- Made sure Power Settings were on High Performance.

Specs:
Intel Core2Quad Q8300 - LGA 775
Gigabyte EP45-UD3LR
Bios : Award Software International F7 / 08 / 31 / 2009
8 GB RAM - PC2-6400 (400 MHz)
Geforce GTS 250 , 1 GB VRAM, 256 bits Bus-width

What do I do?
How do I isolate this issue, aside from going to a friend and replacing every piece one by one as that would take huge amounts of time?

Any ideas?

Thank you.

LE: Any kind of FFT test in Prime95 leads to the system freezing up.

I'm assuming it's between the CPU and the Power Supply?

Is the inside of your case clean? All cooling fans spinning normally? CPU and GPU heat sinks free of dust buildup? Check the power supply fan for dust too. The last time I had artifacts on screen it was my Video Card failing. It just got worse and worse until I finally had to replace it. If something is iffy, stressing it will often push it over the edge. I can't say for certain its your video card, but it would be my prime suspect.