Unavailable at this time

I have been having problems lately with a lot of channels being currently unavailable. I do not watch TV everyday so I am not sure how often this problem is actually taking place. After the TV has been on for a while a few channels start coming in. There is no consistency on which channels work or when. HD and regular channels are both effected. Already reset the cable box and checked the cable that plugs in. Are there any other remedies I could try to fix this problem? Thanks.