Sprinkler question

I have lawn genie valve on a RainBird clock. The valve can be operated with the manual screw, and it will open and close. Sometimes. On the clock, it will open and close. Sometimes. Other times, it stays at a state of freeflow, even after the manual screw or timer has been turned off. I changed the diaphragm, and found a snail shell inside. There was also other various crud from our ag water supply. The filter goes in tomorrow. I switched to another actuator, which I think is new. This is 36v, I'm assuming AC. But I fried my tester last week, and haven't got a new one yet. Ideas on diagnosing this? It's a PITA to remove because I have to dig it up entirely to remove the valve, but this time, I shall install new valve WITH union for future problems.
Any help appreciated. It's driving me nuts.
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