I have been trying to troubleshoot this problem for a week now! I have a 1/2 HP, 10 GPM, submersible in a well just for my sprinkler system. The static level is around 28 feet, the well depth from ground level is about 40 feet. I have about 32 feet of 1 1/4" poly drop pipe and then the 3 feet of the pump. Here is the problem - when the system starts up, the pressure reads steady at 35 and the heads come up and work fine for about one minute, then I hear a gurgle from the well and the pressure drops to about 15. I shut the main valve (before the station valves) and it builds pressure for about 10 seconds, I open the valve and the problem starts all over again. I have replaced the drop pipe, the pump and motor, and tried three different lengths of drop pipe but the trouble continues. Any ideas? [:-(]