From what I have been reading from all these threads, I keep thinking that the problem seems to be with the multiport valve.

If it is the type that you pull up and push down for backwashing, one of the seals may be worn out.

With the Hayward Multiport type, the seal between the casing and the top may be leaking, allowing air to be drawn in.

Just my thoughts.

(Poconos. I still like to see a check valve downstream of the pump. It protects the pump seal and the motor. Standard engineering practice.)

Pat