PC starts lagging after 15-20 mins.

[TheWatchingGamer]

Hello everyone.
So i am asking maybe a common question but I can't seem to find an answer to it on google. So my question is why my pc starts lagging after like 10-30 minutes? Even when i just turn it on and leave it to work it starts lagging. Also when I go to resource monitor CPU usage is around 20-30 % but frequency goes up to 101% when it starts lagging and stays there until restart.
Things I tried:
* Defragmentation.
* Cleaned the case from dust (except PSU less than other components (afraid of taking it out))
* I also cleaned my pc from registry and all that stuff.

PC specs:
CPU: Intel core i5 3350P
Graphics card: GTX 650 1gb
Ram: 8gb
Motherboard: Gigabyte B75M-D3V
PSU: Codegen 500W Model 500X
Hard drive: 1 TB

Thank you in advance for your answers. Please help :/

 

[notherdude]

you may have malware - run malwarbytes http://www.malwarebytes.org/mwb-download/

 

[TheWatchingGamer]

you may have malware - run malwarbytes http://www.malwarebytes.org/mwb-download/

Ok i'll try to be sure, but i doubt a virus is the problem here. But hoping it is because I wouldn't want to find out that I need to change my psu... (money problems :/)

 

[notherdude]

Find out exactly what process or program is using up all that CPU - ctlr+alt+del > task manager> processes>cpu

 

[TheWatchingGamer]

So I did a full system scan with malwarebytes and my avast antivirus but none of them found any viruses. However, now it seems to be working as always. No lag or anything. It is strange though. I hope it's all ok now even though i did not find a reason. And thanks notherdude for your help!