Sounds to me like a synchronising issue - definitely a sharing/control matter. Speed variation in gennys? Hmm. If a diesel set hunts its generally an injection pump governor problem, or the same for petrol but on the carb. However, this would result in cyclic problems and a host of other evils - and dropping like flies of breakers.
Several years ago I got up one morning to find the lights fluctuating from about 50% to full on roughly 2.5cps. The tv didn't like it at all!!
This lasted about ten minutes before we had a 30 second outage then back to normal. A few days later I saw our chief utility troubleshooter in the village and asked him what it was about. His reply was that it was something that theoretically couldn't happen; the ( at that time ) two alternators supplying our region ( we were disconned from the national grid for repairs over the weekend ) had got out of sync with each other; the control system had crashed. I never stopped to work it out, but I guess it is theoretically possible, especially if balancing/motive force is not up to scratch.
Call up the load-box suppliers