Low pressure resolved
The pros showed up this morning and were able to diagnosis and repair my problem today. There was a pressure reducer, but it was buried (not in a box) between the backflow assembly and the meter. There was so little room between those two boxes, I guess the original installers decided they couldn't fit a box in the space. They buried the pressure reducer and it was completely corroded. When the repairman touched it, it fell apart. I must have been losing pressure for a long time. It finally gave out completely last Wed. (letting only a trickle of water through). They also believed that the backflow assembly was not working right (old and somewhat corroded - probably not letting all of the water through). Replacing the backflow assembly and pressure regulartor meant also having to replace the meter and the master solenoid. I really didn't understand why, but they said they could not guarantee the work without a complete replacement of the system. Also, all parts were screwed together and could not be undone one at a time. they had to cut the PVC at a point beyond the master solenoid and then unscrew the backflow valve assembly from the main water shut-off.
Anyway, it's all done and all stations work. They adjusted all sprayers (the pressure is amazing!) The job was a bit pricey ($1,452), but I was in a time crunch and the only other service I talked to could not come out until sometime in June. I hope I didn't get taken... they said the parts were pretty expensive. I wish they could have reused the meter and master solenoid.
Well, that's the solution to my low pressure problem.