Ok. Another Verizon technician came to the house and did a second round of troubleshooting and found no line faults. He said that in his experience--more than 7 years now with FIOS--when the boxes reboot themselves, it is the set top box that is the problem 99% of the time. Even unstable power does not cause a full reboot--so that rules out unstable power as the cause in my case. Even though we just had Verizon send us a replacement DVR STB a few weeks back, he decided to install a brand new one. He noted that the one Verizon sent a month ago was a reconditioned one. While the odds of getting two boxes in a row that reboot themselves is low, it is possible and he says he has seen several cases where the reconditioned boxes still had gremlins and faults. So he gave us a brand new black DVR. Fingers crossed
The brand new box--the really, honestly, brand new and not reconditioned box--seems to have done the trick. We've gone a whole week without even a hiccup. So my advice to those who suffer the same problem is to insist on a new box and refuse any and all reconditioned ones. No doubt the phone rep will say he/she does not know what will be sent but is sure it will work. If so, refuse the offer and insist that a Verizon Tech visit the house. The techs are all top notch and really do go the extra mile to figure the problem out. It took a while, but the Verizon personal touch did the trick!
Almost a month now and no problems whatsoever. No doubt about it, we had two bad DVR/STB boxes in a row. I'm glad the mystery fault chase is over.