completely confounded by air, prime problems

I have had the champagne bubbles for a while and now, but I didn't have the more serious following problem until recently: my pump strainer basket is almost empty and there is an air bubble in my cartridge filter in the morning after the pump has been off all night.

When I turn off the pump, my solar system drains first and the swg also appears to empty (it is the highest piece of equipment besides the solar). There is a check valve after the heater and before the chlorinator that appears to be operating fine. About 15 minutes later, you can see a bubble at the top of the Jandy check valve after the filter. It SLOWLY gets larger. Once it is large enough to fill about 1/2 of the valve body, air starts bubbling into the filter from somewhere. You can hear it like chugging. Shortly after this point, the pump strainer starts to lose prime. The solar valve is in "closed" position. If I close the ball valve to the solar even at this point, the bubbling stops.

I have tried:
-opening the cartridge filter and replacing the filters. Cleaning the filter and air relief valve thoroughly.
-servicing the Jandy check valve after the filter and replaced its o-ring (although it wasn't leaking).
There is no leaking anywhere on the pad during the day. There is a very short run above-ground on the suction-side of the pump with only a 3-way Jandy never-lube. No obvious leaking there.

The solar company FINALLY came out today to check their system. To his credit, he did not rule out the solar system as the cause. He serviced the solar valve (it was working fine) and checked the roof for leaks and the vacuum breakers for proper operation. All was fine. Interestingly, while he was servicing the solar valve the Jandy check valve at the filter held seal just fine, even with the piping after it wide open. After it was all put back together, he saw the prime losing process, which takes quite a bit of time so good for him to stick around. I did voice to him my concerns about the original install. The Jandy check valve has only about 6-8" of vertical head after it, which is too short. Jandy states that 18" of vertical pipe run should occur after the valve to keep it positively sealed. The solar installers didn't maintain this vertical run when they cut into this run to install the solar feed and return.

I have an engineering background and all this is confusing even to me. The solar guy was stumped--he could not figure out why this should be happening with the ball valve closing the solar feed. The only thing this does is put the drain-back check valve inside the solar valve out of operation.

He has never seen anything like this.

I tried one more thing to test if it is somehow the solar system. With the pump off, I put the solar valve in the off position and closed the ball valve that I have leading to the solar intake. Then I primed the pump and system AGAIN. Then I turned the pump off and left it this way all night. In the morning, the pump was primed beautifully. As soon as I turned the ball valve open in the morning (pump still off), the pump lost prime as follows:
-SWG drains towards pool
-bubble appears in body of check valve at filter output
-bubbles continue to gurgle into the SWG from heater side a few per minute
-bubble grows in body of check valve at filter output
-check valve fails eventually and bubbles start to enter the filter a few per minute.

Where is the water going before the check valve at the filter fails? Maybe it goes back up the intake of the solar? That doesn't make sense! The solar valve is still closed... The swg is clear so I can see at this point it is empty (emptied as soon as I opened the ball valve) and no water is flowing through it.

Is the filter check valve failing earlier than I think and letting water drain back into the pool? I opened it to service it when all of this started happening and everything looked okay.

Help!!!