Rogero, in the last year I faced a problem like yours; the stack was rebooting itself several times without logging any issue. After a couple hours troubleshooting I figured out a little difference between the name of tha config file of a unit; after equalize ALL the file names of the .cfg files the problem stopped.
Actually I don't know if this case is exactly the same as you are facing, but if all the switches are running the latest versions and are with the unit IDs fixed, I cannot think in anything else.
Regards