High P2P on the co-ax's can cause random reboots on some DVR's.
If it was being 'hacked' why would it still reboot with the LAN disconnected.
You have tried the PSU which would be my first thought, after that maybe HDD issues.
Factory default the machine and format the HDD, see if that fixes it?
Not played with that model Quantum so I am thinking generic issues here.