Where are they disappearing from? The set's channel list, the cable schedule channel, or they are still listed but don't tune in when you go to that channel?

Maybe read the manual and see if there is anything in there about it automatically updating or rescanning by itself.

I should have been more specific: the channel name appears when the tv is tuned to that station, but nothing appears on the screen. Also, after a channel scan, channels are displayed with numbers that don't exist; i.e.; channel 34 would appear as 34, then 34.101, etc.

Do you have actual cable service or an antenna. If cable, your provider may require an actual box, even for basic service. If antenna, I'm wondering if those channels might be at the fringe of what you can receive. Is it the same channels or group of channels, or completely random?

I seem to recall something similar, years back when I had basic cable service (no box). My "new" digital TV would pick up some channels but then "loose" them hours or days later. Then others would appear and then disappear. I think it had something to do with how the cable operator scrambles the signal.