I just installed a shallow well pump for irrigation. I live in Florida. I am down 25 feet and have a new 3/4 hp shallow well jet pump with a new 5 gallon pressure tank. I hooked it up to an existing irrigation system that had been on city water. The pump primed immediately and clean water immediately followed. I turned on the irrigation system and zone one worked perfectly as the pressure remained at about 25 psi. Then it changed to zone two (with fewer sprinkler heads). The pressure dropped to 10psi and the sprinkler heads just dribbled. Same with zone three. Then zone four worked perfect with all heads working and the pressure back up to 25. Zone one and four work about 80% of the time and have the low pressure problem about 20%. Zones two and three have low pressure 100% of the time. Even running the hose only, (thereby bypassing the valves) there is good pressure 80% and the low pressure problem 20% of the time. I am very confused. Any advice would be appreciated.