Okay, several causes (most already listed)

The main one is overheating. That's a biggie and will cause reboots a lot.
Take the side / top off and point a fan at it (seriously!) and work like that for a while, see if it's still rebooting. If it is, chances are overheating isn't your problem.
Memory is also a thing, if you are able to pull out sticks, then try them one by one (or pair by pair, if that's how your PC is working) But be sure to GROUND yourself before going into the innards. (Touching the case while doing it is usually enough.)

If it's faulty memory, send the thing back (if you are able)

Silke