Solved PC freezes causing CPU temp to rise high.

Hello.
Of late I have been having an annoying problem with my PC. It is a self build from 2012.
Core i5 2320 on a Gigabyte board (GA-H61M-D2H-USB3)
O/S is Windows 10 (version 21H2) on a120GB SSD 8GB Ram (2 x 4gb)
Most of the time it runs fine but then for no reason it will just stop loading a web page, it will loose connection to the internet (wired) and then the CPU temp will rise, well into the 90%C (I have installed CoreTemp to monitor the CPU temperature). The CPU temperature only starts to rise after the PC locks up. Once, the PC just shut down then restarted, as it was rebooting it sounded the over-heating alarm and shut down again. It has only done this once (shutting down). Mostly the PC just locks up and drops internet connection - I then reboot and it's ok - untill it happens again - which could be up to a week or so later. I have not known it to do this more than twice in any one day. Most of the time it runs fine.
I have taken the PC apart, cleaned out the case, motherboard, Ram slotts, fans etc (there is no graphics card) and I re-installed windows. But it's still doing it. I would be gratefull for any ideas anyone has as to what could be the problem.

Many thanks for your reply.
No I haven't replaced the thermal paste on the cpu. The problem occurs before a rise in CPU temp. Would a poor thermal paste contact cause the problem before a rise in temp?
I'll give it a go. It would be a cheap fix and if it doesn't work then that is one more thing I can rule out. Thanks again.
I forgot to add to the specs in my initial post - my PSU details:
Trust PowerMaster 570W Pro Dual Fan PW-5500. Which I bought in 2006 - and when I opened it up the other day it was full of dust!

Just an update:
I have decided to take the opportunity to replace the CPU cooler, rather than just reapply thermal paste. The cooler at the moment is the original one that came with the i5 - over 12 years ago and it's a bit noisy.
I will be doing this today. And then see how it goes. I'll update in a week. If it's still a problem I'll do the "speccy" thing.
Many thanks again.

Ok. I will do that. I will give it 7 - 10 days after I fit the new cooler and post back, unless it locks up sooner - I dont think it has gone more than 7 days before it locks up, on average it's about 2 or 3 days.
Thanks again.

Hello.
Everything has been running fine since I replaced the cooler. Things have been staying sub 70c when in use (avg room temp 23c) - idle 35C to 40c. I noticed when removing the old cooler that one of the four legs that held the cooler to the M/board was not pulling the cooler tight to the board - I could see slight movment on that corner. They were the "push down and twist" type of fitting. It would not tighten up when I tried it.
So, so far so good.
Thanks again.