Hi Measter,
In the absence of anyone with brains i'll tell you what my first steps would be
First check that you dont have a build up of dust in your computor and that it is not overheating.
Check that it is not a memory problem, you can check your memory by visiting here and using their tester
http://simmtester.com./
It may be that your graphics card drivers have gone a touch wrong so see if there is any updates for it.
That should keep you busy until someone who knows what they are talking about comes along