Flow from my pond pump decreases significantly over one or two days. It appears to be a result of air building up in the pump or the pump discharge. Any experience or advice?

Quick description: I have a 2,000 gallon lined pond in south Florida. The pond was built about 1.5 years ago; therefore, it has established a good amount of natural organic flora and fauna. It has a Pondmaster Hy-Drive 1600 gph pump submerged in the pond (submerged about 2 feet). The pump sends water up and out of the pond, then back down to a short length of buried pipe, then out of a waterfall - where the water flows by gravity back to the pond. I put an "air release" valve at the high point of the discharge just in case of air entrapment.

The pump worked fine for many months, then I had trouble with snails clogging the pump impellers and impeding flow. I think I solved that by putting some filter material around the pump. But then, over time I started noticing that the flow would decrease from normal flow to almost no flow. Eventually it would only take a couple days to go from normal flow to very little flow. I noticed that when I back flushed the pump a BUNCH of air would come bubbling out. I did NOT notice any air coming out of my air release valve in the discharge piping. After flushing out the air, the pump flow would return to normal - only to repeat the cycle a couple of days later.

Therefore, it appears that air is getting entrained in the pump or in a short section of discharge pipe near the pump. But I don't know how and I don't know why. Is it possible that air released by the algae and muck in the pond is the source? Is the air being released from solution by the decrease in pressure at the impellers? I'm at a loss. Any advice would be much appreciated.