Screen/graphics problem
 

when i left harbor city, the screen wasn't right, it looked 'block(y)'; i re-started the game, then it was ok; in 2 hours i noticed dark squares on some parts of the ground; i re-started the game again, it was ok again ? i've been playing 2 weeks, i haven't noticed any graphics problem, until today; thanks

Could be a hardware problem. Overheating is always a good candidate.

Agree - overheating shows artifacts like you describe.

Might be a good time to make sure all the fans on your PC are clean.