We've been running an HDHR receiver with a dual tuner here at the studio receiving the main transmitter for the past 24 hours. So far I'm not seeing the problem with periodic unlock or SNR drop, but we'll keep this monitoring system up here for a while (We usually use this system for field testing).
For the viewer in Johnstown, he may be receiving our translator on channel 21 which receives channel 12 and retransmits the signal without demodulating down to baseband. This method preserves the exact modulated data as the main channel, so any errors would show up on that transmitter as well as the main channel 12 transmitter.
One thing I should mention is that with the bouts of severe weather we've received this season, antenna and feed line damage is something you may want to rule out when having reception problems. This is true even if only having a problem with one or two stations. Antenna problems, unterminated lines, and amps can all cause problems that may not affect all channels.
Regarding amplifiers... If DTV signals get clipped or distorted due to high levels passing through the amplifier, the stronger signals can often become unreceivable. If you have an adjustable gain amplifier try different gain settings to see if that makes any change.
Director of Engineering
WNYT-TV / DT
715 N Pearl St. | Albany, NY 12204