50°C as shown by psensor app.
<<Are you sure it completely locks up?
No, if I don't wait for ~20 minutes or more for mouse/keyboard to be activated, I reboot after ~5 minutes of "frozen" state.
CTRL+ALT+F1 brings to the system concole normally
Reinstalling is a lot of work but might help. If you don't care for LTS versions, perhaps try a newer one. But that is all based on guesses, maybe ask in a forum specifically for Ubuntu and tell them about the specific hardware you are using. Perhaps someone has similar experiences.