Here is my problem I have 65 psi staic pressure but less than 20 psi at the spinkler head. What have I done wrong? I have already double checked for leaks. So whats next.

I sure could use some help troubleshooting an irrigation system. Please forgive a long explanation... The system is about 10 years old, located in coastal SC. It's fed by a shallow well, and includes 4 zones, 2 of them use Rainbird 1600 spray heads, the other 2 use Orbit Voyager II Professional gear-drive rotor heads. There's been no major issues in the past, just the usual broken heads, clogged filters, bad pressure switch, etc. The system had been off for about 10 days, due to frequent rain. When I started it up this week, two of the zones caused the pump to cycle on and off immediately due to high pressure. The pump pressure gauge was replaced at the start of this season and confirmed the high pressure cut-off (50psi) was being reached. One of the problem zones used Rainbird sprayers. I was able to get it going by replacing a few dirty filters, although the pressure remains near the upper limit. The remaining problem zone contains 5 Orbit rotor heads. Two of them were replaced this Spring, and the other 3 are a little less than 2 years old. I removed the last head and flushed the line. The pump ran smoothly at very low pressure, as expected. As soon as I replaced the head, it immediately resumed cycling on and off. This sounds like a small number of heads for a single zone, but it is more than sufficient to water the area involved and has not been a problem in the past 10 years. Since the pressure fell very low when I flushed the zone, I made the assumption (good or bad) that there is not a problem with the valve. The builder installed the system, and unfortunately there's no info on where the valves are located. I've searched off and on for them, but have been able to locate only 1 valve, and it's not related to this zone. Is that a stupid assumption to make, under the circumstances? We have had an unusually wet Summer. Since the problem occurred simultaneously in two zones, what's the likelihood that it is related to a higher than usual water table? If that's the problem, how can I temporarily get the pressure down on this zone without installing more heads? Any other ideas or suggestions? Thanks very much for your help.