I had a similar experience where an older Tivo on Comcast started hanging and rebooting a lot. I replaced its hard drive from Weak Knees as suggested, but that still didn't do the trick, unfortunately. I ended up shelling out for the newest top-of-the-line Tivo unit that they had for sale at the time, and it's been working great since then. Cable card swapped right over, the phone call to Comcast to fix up the cable card authorization was quick and painless.

The siren song of cord cutting from Comcast is strong, though, and getting stronger every year.