Are you using an out-of-band modulator to communicate with set tops? Motorola's OM transmits at 75 MHz, between channels 4 and 5. If the output level is too high, it will cause problems on channels 4 and 5, and can drop beats at random in the higher channels. Just trying to help.
This is CABL.com posting #147417. Tiny Link: cabl.co/mMvR