Dell XPS 7590 keeps rebooting

I have a Dell XPS 15 7590. I bought it 3.5 years ago. warranty expired.
It suddenly in the middle of document editing in Windows restarted itself (or shut down, don't remember), and then it keeps restarting after a couple of seconds!! It doesn't boot to Windows.

When I press F12 and do the Dell diagnostic, it goes on and at some point, it restarts again! If it's unused for a while and I do the diagnostic, it will go to around the end of it but again restarts. Once it could make it to the end and there were NO issues! However, it asks whether I want to do a deeper diagnosis (or sth like that) I confirm then in the middle of that restarted! If is used for a while it restarts in the beginning of the Diagnostic! Having these facts in mind I suspected that the problem could be high temperature. So, I checked fans and they were OK. Then I changed the thermal paste on the CPU and GPU. In fact, after, this it could make it through diagnostics as I described above but couldn't make it to the second diagnostic and also the repetitive restarting issue remains.

This is SO absurd that this laptop which I bought for around 3000 Euros has this wired issue only after 3 years. I had another laptop for 11 years and didn't have these issues. I cannot work because of this issue!

Anyone can help??