A common reason for the intermittent disappearance and reappearance of DTV reception is caused by multipath, or marginal system bandwidth problems at those particular frequencies. Here are some things to check on your end:
1. Coax: How old is the coax between the antenna, any splitters, and your TV? Coax, especially when exposed to sun or heat over time, will allow signal leakage or change impedance, which compromises bandwidth, specially for UHF DTV channels. For the same reasons, is your coax cable running through a closed window or door that has squashed it over time? Also, is that coax a good quality RG6, and NOT RG59? RG59 is not suitable for use with reliable OTA DTV reception, but may have been already in the house from previous cable or analog TV days.
2. Splitters: If you use cable splitters for multiple TV's, make sure they are rated to 2Ghz and not from Radio Shack. Old splitters are rated to 470Mhz (down hill with a tail wind), and are not suitable for modern DTV use. Also terminate all unused spigots on the splitter with 75 ohm terminating resistors. Un-terminated connections will alter the bandwidth and impedance of all distribution points, making for marginal bandwidth and potential signal leakage.
3. Height of antenna: Not always possible, but an outdoor antenna at least 30 feet off the ground, gives you the best opportunity for watching OTA DTV. Antennas in attics, not so good.