Well re-applying thermal paste is a big matter. I’d try stressing it and seeing if you can force it after cleaning the vents.
So do things to stress the processor and see if you can force it to happen repeatedly. If it is a thermal issue you should be able to cause it by inducing high load. If you can’t it points more towards other issues. A fault in RAM or the motherboard for example, a loose module or connection, etc.
And just FYI if you need more help in future this community isn’t intended for it. Try posting in a tech or computer help community for a better chance more people engage.