pool pump dropping flow rates mystery
I am a long time reader and fan, first time poster. I am facing a puzzling loss of flow in the pool circulation. The setup has just the pool with solar heating. It is about 30K gallons, with an Intelliflo pump and hayward 400sqft cartridge filter.
There is a check valve right after the filter and another one right after the solar panels. There is a vacuum breaker at the top of the panels.
Normally the pressure gauge at the filter reads 8-10psi, and the flowmeter installed after the filter and checkvalve, but before the solar panels reads 50-60 gpm. Solar is engaged and operating normally, pump is at 2650rpm as programmed. System is relatively quiet.
Once in about 3 days, when I take a look, I find:
flow is down to 15-25 gpm and fluctuating.
there are bubbles in the pool where water is being returned to it.
pressure gauge on the filter reads 25-30psi.
there are gushing noises. Relatively loud, but I am not able to pinpoint where they are coming from. If I had to guess, I'd guess somewhere near the check valve that is right after the filter. The valve body is completely opaque, so can't see anything here.
the check valve after the solar panels has a clear lid, and I can see the flowing water is not filling the whole pipe and there is turbulence. All piping is 2", but normally the pipe is full of water, and no turbulence is visible here.
There is 3-6 drops per minute leaking from the inlet port of the filter.
From the bubbles, I guess, air is entering the system somewhere. But I don't understand first, why it happens only some of the times, and second, how to explain the readings of the filter pressure gauge and the flow meter.