Reboot on power-down
 

Title says it all. Happens regardless if I shut-down with a long-press on the power switch or a short press followed by power down off the menu. Only seems to happen after 'long' [several hour] usage. If I try to reproduce [i.e., after a short session], it always shuts down gracefully. Any clues? [brings back memories of some particularly bad releases of windows....]

Thanks!