I recently noticed that my rotary lawn sprinklers are no longer spraying as far as they used to, and the spray pattern from each nozzle has gone from the typical "spray" to more of a singular stream. I have a 12-zone system, with 3-4 sprinklers on each zone. Most sprinkler nozzles are 4.0 gpm. When the system first kicks on, I have full pressure and performance, but after a few minutes it significantly drops off. [deep well, 20-year old submersible water pump, D Squared 40/60 pressure switch, 44 gallon WX-250 pressure tank] In a failed effort to compensate, I adjusted the pressure cut off up from 30/50 to 45/65, and I pumped up the pressure tank to 43 psi. When the sprinklers are not running, the system pumps up from the cut in at 45 psi to the cut off at 65 psi within a minute or so. When the sprinklers are running however, the system runs down to about 18 psi within a few minutes and stays there until the sprinklers turn off (at which point it quickly climbs back up to 65 psi). I also noticed that within the past week the radius on the sprinklers that I've watched the closest has shrunk down by another 1-2 ft, so the problem is potentially getting worse. (Also...I'm not sure if it matters, but the needle on the pressure gauge vibrates quite a bit as the pressure tank fills.) Is my well pump going bad, or does this behavior indicate that the well cannot supply enough water to the pump?