Laptop switching off every few hours

My computer has a/more problems. The thing is always switching off after a few hours of work. Simply wents black and (sometimes) you have to wait for a few minutes until you can restart. I was thinking it may have to do with the heat. Now the graphic card runs with half power and the cpu runs also with 1200 mhz instead of 1500 but the problem did not disappear.

Today for example the system went black after 20 minutes of work. Yesterday after 4 hours....

I guess it is time for a new computer. This one is already 3,5 years old.