That problem was traced to two return lasers which were being combined at the headend. Normally, the two were different enough in wavelength to avoid any in-band beats, but once in a while, the two wavelengths would drift close enough to cause problems. Problems could very well surface later. If lasers are combined, make sure they are specified to always have different wavelengths. That still will result in normal noise combining, but is acceptable if the noise is low enough.
Confusion still exists as to how to set drive levels in return plant.
The headend receiver measures the level of the received signal, and sends a command to the home transmitter to increase or decrease its level until the signal received reaches the proper level. This keeps all headend signal levels correct, but doesn t mean a plant is operating at the correct level. The proper level may or may not be adjustable within the system. If it is adjustable, it needs to be set so all components of the return plant are operating correctly. If it is not, then the signal delivered to the system needs to be adjusted, so the return plant is working at the correct level.