What about the local channels like 35.1, 47.1 and 62.1 ? They are also intermittent to no lock for me as well which is kind of strange since these are from a local transmitter. I have never seen such poor reception since I updated my antenna in 2009. 15.1 is only watchable only around 70% of the time.I'll have to contact Vermont PBS to see if they're having transmitter issues, it seems likely. I've also had Mountain Lake PBS 57.1 go from a very strong signal (at least since the switch-over) to unwatchable and often no lock even. On the other hand, 62.1 (CityTV), which used to be one of the weakest channels, has lately been very strong and stable, up to 80% signal strength!
I highly suspect the bank of Hydro meters installed in my building (just mere feet behind my antenna!) have been causing antenna interference issues. Since dozens were installed a few years back, I now get bursts of errors if I have my UHF or VHF amplification turned up. It could also be someone's WIFI, but since I've had to push up the VHF power gain for WVNY-22 lately, it's interfering with my other channels.