Recent Tivo Series 2 lockups

Hi,
I have two series 2 single tuner tivos. One of them has recently begun to lock up for no apparent reason. It only happens once in a while, but when it does happen, it is locked up hard. The screen is frozen and the only thing I can do is unplug and replug the TiVo to get it working again. I'm not exactly sure when it happens, but I'm guessing it is in the middle of the night or the morning hours, because it's fine at night when I'm watching it, but the next time I watch after going to bed, it's locked up.

Any ideas what could be happening with it? If anyone needs more information to diagnose, I'd be happy to provide it.

Thanks