Do you live in the northern hemisphere? Do you got summertime right now? Do you recognize higher temperatures in your room than during the rest of the year?
Sounds like some parts are overheating. Such constant overheating can cause parts of the hardware to break. Especially the memory on video cards isn't always cooled properly. Sometimes it is a single memory module only.
Right now I play World of Tanks with reduced video settings. I get about 50 fps. If I raise the video settings forcing it to run on 25 fps, the card is overheating and crashes my whole computer.
Probably one of the memory modules on my card is broken. It gives me artefacts sometimes. Luckily they pop up only for a split second. I barely notice them. It seems to depend what picture informations are stored in that particular memory module.
However at the beginning of this summer, for once the artefacts didnt pop up for a split second, but stayed locked on the screen. I tabbed out, tabbed in again and they were gone. I even made ascreenshot.
The card likes to overheat on Arma II as well. Did you ever knew that Arma remembers footsteps?
Not to mention this bullshit:
When my videocard overheats in FH2, it is very subtle. I only notice when aiming.
And the console looks like this then:
Only gets worse when the wounded shader comes up:
I tab out and in again and the problem has been solved. Overall FH2 is very grateful when overheating my card.
A few years ago my GeForce2TI 64MB started doing weird artefacts a short time after I had started playing games. Looked similar to this:
Tuned out that some Elkos were broken on my mainboard then. It looked like this:
So my first suggestion is that you clean your fans and air-inlets. If that doesn't solve the problem, try lowering the graphics. If that doesn't help, you'll have to start replacing hardware.
Edit:// Bonus picture: