Ubuntu 20.04 LTS : while working on the system is poweroff's abruptly cant diagnose the reason

Recently my system shutdowns in different timings it wont lasts 1 hr it shutdowns abruptly
i can’t figure out the issue. I tried the following

installed lm-sensors package to find out while usage how much temperature it goes.

using journalctl -b -1 -ex flags read the system log to check the issue but still not able to find the reason

Check for the running processes.

If any huge process which takes cpu/memory can kill the system.

check for the free storage too.

Free storage :

hariharan@hariharan-pc:~$ df  -h /
Filesystem Size Used Avail Use% Mounted on
/dev/sda5 38G 16G 22G 43% /

There is no big such process is running

In my current observation

CPU Temperature exceed 70 then only it turned off again and again.

I suspect two possiblity my cpu’s thermal paste totally finished not able to transfer much heat from the cpu through the air vent.
another one is fan is running slower need to be replaced if thermal paste change is not also maintain the temperature.

From the initial use i have not changed the thermal paste no service is given.