Pump doesn't have any pressure anymore, any ideas why?

Here is my situation, I have an inground pool thats about 12000 gallons by my calculations. I have a Hayward variable speed pump, and a Pentair CC150 cartridge filter. The pump is 2 years old now. I don't have any automatic robots or anything, I hook up a hose and vacuum to the suction side to clean the bottom when needed. Everything has been working great since I had the equipment installed 2 years ago.

However, last week I noticed a considerable drop in pressure, and the pump strainer kept getting air in it, like there was a blockage in the pipe and the pump wasn't getting enough water. So I did all the normal steps that I have read to take: checked the o-rings for leaks and they looked good. Cleaned the filter and strainer basket, check the scupper basket, and check the impeller for any debris and there was nothing out of the ordinary. I also ran a snake through the pipe from the pump to the pool and there was nothing that I saw or removed, and running the hose both ways through the pipe didn't dislodge anything. The only thing I can think of checking that I haven't is the actual pipe for cracks, but read on for more info.

The installer put ball valves in the feeder pipe before the pump, and after the filter. When the pump is running at maximum and it is struggling to prime, if I close the pressure side ball valve half way to restrict flow to the pool then everything seems to be ok, the pump stops struggling and the pump strainer get full with water. Except, the pressure is still really weak, from 30-35psi before the problems started to around 10psi now. Opening the pressure side ball valve makes it cavitate again and loss of all pressure.

I'm at a loss for what the problem is. It just happened a week ago without warning, there was no event that stands out that I can point to and say that is what happend. There are no bubbles going into the pool to indicate an air leak, and the water level hasn't dropped to indicate a leak somewhere. Any ideas or advice would be greatly appreciated. Thanks, Kyle