The first thing I would try is disconnect all peripherals (printer, Hard Disks, Graphics Tablets etc. etc. and see if the problem persists. If not then I would add them back one by one to see which is causing the problem.
If the problem remains I would then try creating a new user Mac OS X 10.6 Help: Creating a new user account and see if the problem happens under this new user.
If it does I suggest that you run a hardware diagnostic by inserting original disks that came with your computer and rebooting holding down the "D" key. Newer Macs have Apple Hardware Test located on a hidden partition on the hard drive, in which case you can restart with the "D" key pressed and you'll launch AHT.
Hope it works out.
Hi, I did a Apple Hardware test (better, a Apple Service Diagnostic test which last 5-6 hours!) and nothing was found! but I'm happy for that.
I tried also to use the pc without peripherals attached to it but no success!!!
At last I found out that restarting my iMac every night the problems went away so I'm thinking about a software related issue, I guess the best thing to do should be reinstall the entire operating system from scratch (taking in consideration the opportunity to switch on Lion) but now I can't waste time so I scheduled an automatic restart every morning at 07:30 AM (with the Lingon application) and all works fine.
Thanks for the suggestions.