I have a perplexing problem with my Hunter underground sprinkler system. I have a total of 23 zones including 2 drip lines and 21 "regular" zones, mostly consisting of 4 pop-up heads. Water is fed to my house via a well for which a new 1.5 HP variable speed pump was installed last year. Pressure at the tank is a fairly stead 50-60 LBS.

When a program starts (I have four programs) the first zone always gets excellent pressure. As the system cycles through the zones the pressure decreases, after 4 zones there is very little pressure and sometimes the heads won't pop up. If I turn off the system on the zone with little pressure and immediately start that same zone manually it is fine, great pressure. This happens on all programs and regardless of where the system is at, if I just shut it off/restart it manually in a particular zone, that zone gets excellent pressure. Each of my pop-up zones runs 25 mins. I have had two sprinkler companies look at the system and the problem continues. Does anyone have any ideas as to what might be going on?