What kind of antivirus and operating system are you running? Could be malware running in background using your computer for nefarious purposes.
Run antimalwarebytes free, spybot free...
This made me think of Antivirus programs can use up a lot of memory.
I had a anti-virus program on an old computer that had “Real Time” virus monitoring. The antivirus program is basically running continuously whenever the computer is running.
That antivirus program pretty much used up my CPU. So, I turned off the real time feature and eventually uninstalled the antivirus. I was better off with out it.
Your problem sounds a lot like what I had with the antivirus program.
Win10, v1803, Build 17134.165.
Webroot v9.