Black TV Screen above Channel 46

Question

After my TV has been on for a couple of hours all channels 47-125 black-out. If I unplug the set or simply leave it off for a couple of hours they come back - only to disappear after running for awhilelonger. I switched the cable input from Ant 1 to Ant 2 and this seemed to solve the problem for about three weeks - but am back to this on/again - off/again routine. The lower channels are unaffected.