I saw this once on the computer of my wife, just after login all normal, but after a while some process using almost every CPU cycle and memory it could get. Whatever I tried, I never found what caused it.
My way out was creating a new account, transferring all files to it and deleting the old one.
Not very satisfactory, but it worked.
You may give it a try when all other attempts fail.
Hope I still added something useful and helpful
Henk