Well pressure problem

So I am having a problem with my well maintaining pressure with my sprinkler system on. It seems to keep up for awhile but then falls off. There is only 3-4 sprinkler heads per circuit and they run for 8 to 10 minutes. The well is 12 gpm @430ft but the pump was set @ 300ft because the static level was @ 40 ft. Not sure if I am running the water level down to low or there is some sort of other problem? Anyone have an idea? This well/equipment is only 6 years old!

First you need to make sure the pump isn't just cycling on and off while the sprinklers are running. That will make is act like you say, as sometimes the pressure is at 60 PSI, and sometimes it is at 40 PSI.

If the pump is running steady and the pressure falls off, the water level maybe deeper than usual if the well production is off, you may have a hole in the drop pipe, or the pump is worn down and not producing as much as it should.