Latitude E6520 freezes and shuts down randomly

In the last few days first time last sunday) it occurred 4 times: the laptop freezes for a couple of seconds and suddenly shuts down like the power was off (but it was connected to the power and also the battery was charged). No errors are reported in the logs. The battery has been changed not too much time ago. All the Dell PC diagnostics are passed successfully.

At first, my idea wsa that the problem was related to the temperature, I monitored the temperature and I found that under stress the CPU reached 90 degrees C. Then I used the laptop only with an external fan and I checked that the CPU temperature was less then 50-55 degrees (hard disk was under 35), but it happened again.

I am very worried, what can I do?