Pressure drop in last zone

[Mitcheal]

Hello, I have a 5 zone system. All zones are working great except the last one. It will start with normal pressure, and then drop to about 20psi after about 15 min. I've taken apart the valve, no issue that I can see. I've replaced two of the sprinkler heads with lower GPM versions and still have the issue. I've verified I am below the max GPM of the pump.

This is a shallow well pump in North East Florida. If zone 4 has a bad valve that does not close completely, would that not cause an immediate low pressure, not after 15 min? I'm at a loss here and getting frustrated. Any help would be appreciated...

Thanks.