It's probably one of three things
- Operator - improper adjustment, it can take a bit to find that sweet spot, you can sometimes adjust it close to the point walk away only to hear it slurping air a couple minutes later. Then you over compensate in the other direction, always missing the actual point.
- Air draw - flow rate is too high for primary pipe in overflow height. The higher the flow rate and the smaller the pipe diameter the higher the intake velocity. The higher the intake velocity the more space you need from the top of primary pipe to operating water level in the overflow. If the space is inadequate you'll gnerate the occasion vortex with brings air into the line which messes up the flow rate.
- Inconsistent return flow rate - a few things can cause this but your return pump has to pump a consistent flow rate. Obstructions at the pump intake, air bubbles, poor quality pump are all examples.