do you think it is related to Montery OS? I am also having the same issues

this is exactly what is happening to mine 2015 macbook pro 15inch also. I only have not tried to repaste the cpu as the fan and board and they look still very clean.. It is really frustrating and hope apple or someone really can solve this issue. I think this is a design flaw from apple.

I have repasted the CPU, GPU and cleaned again the fans.. still the same... it is really frustrating and I am thinking selling and get rid of macbook..!!