Don't know. Mysterious. Perhaps there is some kind or residue or dirt caught in the pressure regulator valve. Try opening it all the way and closing it a couple of times. Exactly what pressure are you getting?
Does the pressure actually increase even when the nozzles are spraying? Are you using extra small nozzles?
Also, check that the tube that shunts the excess flow back to the tank is working OK and not blocked or pinched. Is the tank agitation powerful?
Is there a way to measure or watch the flow entering the tank? Weak flow here means that the majority of the flow is going to the nozzles. Are you getting excess gallons per minute?
Check the system voltage; 14.5 is about the maximum you want. Do you happen to have an extra strong charging system? 16 volts?
Had similar problems at about the 400 hr point. Bought new pump and no more problems. Might check lines and tips are for sure clean but if all that is clean with no blocks then swap pump. Call steel green guys. They are glad to help and have all parts needed for z spray.