rainwater catchment tank pump cycling on and off

The Grundfos MQ3-45 pump keeps cycling on/off. I've cleaned the four Hunter PGV-075-ASV valves many times however after about two weeks the pump starts cycling on/off again.

Rainwater catchement set-up. Rain flows onto comp shingle roof (potential source of small stones), flows down drainpipe along the side of the house to a 2500 gallon tank. Drain pipe has a leaf filter, and before the water flows into the tank there is a 3' 'first flush' pvc pipe designed to capture debris before water is diverted to 2500 gallon tank. Tank usually fills after first couple of rain storms. In the Spring I turn on the drip irrigation system and power on the pump. When a valve is open the pump detects pressure loss and pumps water. After the scheduled watering completes all valves are closed and the pump stops. However, if a stone or something gets lodged in the valve the valve would remain open and the pump would continue. The pump cycling happens anywhere from every 2 to 4 minutes and lasts only 5-10 seconds so the leak must be small. Whenever I've opened up the valves I don't find any debris whatsoever. I've checked the water tank, sticking a broom handle down to the bottom and haven't found any residue. Also, the outlet on the pump is a couple of inches off the bottom so there would need to be a lot of residue accumulated before flowing out to the pump and valves.

Any ideas or suggestions on how to troubleshoot the pump cycling?

Thanks.

 

Irrigation valves like that are designed to work with clear water. They are not very tolerant of solids in the water. I'd consider installing a filter on the output side of the pump and before the irrigation valves. I'd get the largest filter you can afford to minimize how often the filter needs to be changed. A coarse filter should be good enough and would be good for your drip system too.