Most likely, it is the hard drive going out. 2 things to do while you shop for a replacement drive:
1) Open the case and use compressed air to blow out the dust - if the problem is heat related, this will definitely help.
2) Connect the AC cord to a UPS battery backup device - this is a good idea to protect the unit from any AC power outages. I have them frequently at my house, many are only a 1 second or so outage, but that is enough to cause my DTivo's to reboot.
1) Open the case and use compressed air to blow out the dust - if the problem is heat related, this will definitely help.
2) Connect the AC cord to a UPS battery backup device - this is a good idea to protect the unit from any AC power outages. I have them frequently at my house, many are only a 1 second or so outage, but that is enough to cause my DTivo's to reboot.