Strange shutdown problem on Vista home basic

Hi all, my laptop has been running vista home basic for years with little problem, until recently when it started to shut down just few minutes after booting up. I noticed the system being more highly unresponsive than usual, and when i run something like firefox or a system function like event log, it just completely shuts down with no warning. I went through numerous rounds of trouble shooting and this was the result:

I finally managed to keep the computer running after rebooting to safe mode.

1. i did a thorough virus scan with avast, even using a usb initiated scan. it was clean. same with malwarebytes. I also ran a registry and vista cleanup from ashampoo, and ran chkdsk and there were no problems with the hard disk or system as far as i could tell.

2. i noticed the laptop was hotter than usual, so i suspected overheating. but later i was able to restore the system and kept it running for hours with no issue. so i am thinking overheating is not the problem. I also managed to keep it alive in safe mode, although this only happened after leaving the laptop to cool down.

3. after everything failed, i did a system restore. voila, the shutdown problem stopped for a while. but it happened again after a week, and the only new thing i did on the restored system was to update firefox. I am not sure if that is causing the shutdown, since vista support is now over and there could be compatibility issues causing a system shutdown.

4. i disabled updates for firefox and windows vista to make sure nothing changes on my computer. but then without warning, the shutdown problem came back.

5. as a last resort, i did a complete restore of an archived backup of the C drive from last year which I made after a major cleanup and patch for wannacry from the microsoft website. The shutdown problem stopped with the archived system restore.

The restored system was working for some time before the shutdown problem started again. I did the whole restore again, and this time I completely uninstalled firefox. It seemed to help some, fingers crossed.

Does anyone know what could be causing the shutdown? If a complete restore can solve the problem, does it mean it's a software issue instead of hardware, but why does it come back after a while - ideas? Apart from firefox i havent installed anything new on the machine after the restore, and i have defragged the disk.

I'm really stumped.