I had similar probs just recently, i was convinced it must be air. the flow slowed right down & was banging just as if it had air in it.
It was the pump filter that was blocked up restricting the inlet flow!
Cleaned it & it worked perfect for an hour then same again, blocked. looked in my tank & it was full of tiny bits of algea. Cleaned the filter & tank out & has worked perfect ever since.
Tony