Hya Jan.
It could be alot of things that would cause this but lets start with defragmenting youre HD if you havent done that lately.
Then please download
ATF-Cleaner.
Run ATF-Cleaner:
First check "Select All" , and then remove the tick infront of "History".
Then click Empty Selected.
When you get the "Done Cleaning" message, click OK.
Then install and run
RegSupremePro.
It will want to make a backup of your registry , let it. Once it has finished, click on the Registry Cleaner tab, select Aggressive. When it has completed, click on Select, choose All. Click on Fix, and let it fix everything that it finds.
After that install and run Speedfan:
http://www.almico.com/sfdownload.php
And report back the temperture and voltage readings please.