Good pressure for 30 minutes, then drops off

I have a well that was drilled to 160' with a water level of about 80'. It pulled over 20gpm when drilled. I put in a sprinkler system several years ago, no problem with water pressure (4gpm total per zone). 2 years ago or so, I put in an extra pressure tank for water storage mainly (to keep pump from cycling so much).

I started having issues with good pressure for 10 minutes of continuous running water (sprinklers), then the pressure would drop to about 14-16psi with the pump running. Shut off sprinklers and the pressure would go back to normal. I took out the other pressure tank (started right after I put it in). Still an issue, replaced the pressure switch, and replaced tank to a 80 gallon tank in the last month. It has gotten better (30-35 minutes before drop off).

Specs/info:
1.5hp sub pump
Well pump height is about 140'
1" main into house, about 50 feet from house
When no water usage, pressure stays at a constant pressure (system leaks)
I don't have any problem with air coming thru my system (pump cavitation)

Any help out there before I call the Pump guys?

Thanks
Kevin

Calling the pump guys to lower the pump to 158' might help. If it doesn't you should call a preacher because it sounds like you are pumping the well off.