Low suction, high pressure... HELP!!!

I have a bit of a unique issue that a few of the resident pools guys are stumped on. I just bought this house and this is my first pool. I have a 20,000 gallon in ground pool, a Waterway s600 sand filter and 1hp Hayward pump and Pentair rebel cleaner. When I run my pool on filter setting my psi is 12-14 psi and it runs fine. However when I plug in my creeping cleaner vac after a few minutes my suction reduces to almost nothing and my tank pressure goes up to between 35-40 psi. Even when I unplug the little wondering vac the system doesn't recover, little suction, high filter pressure. I have to turn the pump off for a few seconds then on again and it will prime and work just fine... here is what I have done/know so far

1. I changed all of the sand in the filter, all laterals are in good shape seals are good and nothing is leaking.

2. there is no indication of air being sucked into the pump, once primed the pump basket pod fills with water, any air intrusion would be clearly visible through the clear basket cap I would think.

3. I have no bubbles or fizz coming from any of the return lines during operation with or without the vac inline.

4. I have been into that sand filter twice and there is no indication of buildup or gunk and backwashing doesn't change anything.

5. The vac runs great with no loss of suction when running in the recirculation or waste setting. and the filter gauge tank pressure is at 10-12 psi while running in recirculation mode

6. the multiport valve on the sand filter is brand new and nothing leaks, I have taken it apart and there are no obstructions

7. When the filter and vac in use at the same time the PSI starts out around 18 then slowly builds from there.

8. it seems as though the added load of the vac in combination with running the filter is what causes the issue. But why doesn't it recover when I take the vac off to reduce the load???

If I had an air leak before the pump it should be very apparent through the pump basket pod wouldn't it? there is no sign of air before the pump.

if the leak was after the pump it would be in the form of water right? nothing is leaking

I am at a loss, as is the guy I will have servicing my pool. Any help or advise would be greatly appreciated. Like I said before this is my first pool and I know nothing other than what I have done over the last week.

They say a boat is a hole in the water surrounded by wood in which one throws money into... Im starting to feel that way about the hole in my backyard