+kovi29 I admit that while reading your problem, I initially thought that dust accumulation inside the system might be causing thermal throttling of your CPU. My opinion changed when you wrote "verything comes back to normal for aprox. 3-5 minutes". My first thought is that you have a virus, trojan or malware causing this problem. Am I correct in assuming that you're running Windows XP or 7?
Close all programs and save your work, then run these two utilities; one after the other. They both will most likely reboot your system to clean whatever infection it find. Neither of them will harm any of your files. Run ADWCleaner and close the log file when it's done. Then do the same with Combofix. Once they've both completed, report back here whether the issue is better, worse or the same.
https://www.bleepingcomputer.com/download/adwcleaner/
https://www.bleepingcomputer.com/download/combofix/