The culprit could be dust clogging the CPU fan vent, as well as the effectiveness of the stock thermal paste on your cpu wearing out.
Here's what my CPU heatsink vent looks like after a couple of years of use.
And the shiny cpu.
The final bit is to put a blob of thermal paste right in the centre of the cpu. and then replace the heatsink.
As I write this, an hour or two into the first post application reboot, temps are under control, and looking good.