I've had this problem and I found my solution, although it might not be the same as yours... I had a PH
in my tank a few inches below the surface and it was blowing air out every once in a while... What was happening was the nitrogen bubbles from the sand bed were collecting under the pump and when they had collected to a large enough volume that the bottom surface of the PH
couldn't hold them down anymore they would seep around the sides and get into the impeller and get burped out. Prolly not your problems, but just a thought.
92-gallon corner tank, 100lbs of LR
, 140lbs of sand, 250watt 10,000K MH
, 110watts of actinic PCs, Mag 7 return, custom refugium, AquaC EV180 w/ Mag 5
Female lyretail anthias, eyelash blenny, tomato clown, saddleback clown, firefish goby, 2 sand-sifting stars