quick test would be to just pull the airline from the silencer and with the collection cup off, run the line under water to pull water through the air line and dislodge/flush any heavy obstructions. then just hook it back up and see if it settles down.
if you're doing a heavy skimmer cleaning, while you have it apart, make sure you look into the venturi nipple where the airline attaches at the pump intake.
those little suckers can clog pretty easily with carbonate and dust build up. if they are impeded just a little, the level of the skimmer can rise enough to overflow even though it seems like its pulling enough air. Remember, less air = more water through the skimmer and higher internal levels.
even if the nipple looks free from build up, it doesn't hurt to soak and clean it. I use the non-cutting end of a drill bit to push build up out, then soak and use felt wire from a craft store to do a final cleaning.