Tubes108...In military equipment that I worked on we had a similar situation.... we were adjusting amplitude and phase of an AC power supply to account for variables of the load. The way to do it is to run the adjustment process until the parameter you are adjusting is within some tolerance, (and then a little longer so you don't always get hung up ON that tolerance). Then, quit the adjustment process unless the parameter drifts outside of a larger tolerance. In other words, introduce "hysteresis".
This is how automatic tube bias adjustment onbviously ought to be done, but...? Does anyone know the actual details?