Hi harrywood,

I think it might be hard to diagnose your problem remotely. I believe you have the feel of the system. Your not loosing prime after shut down your foot-valve is okay and keeps water to the pump. You could be drawing down the well below the intake and loosing prime. Getting a local expert on jet pumps might be useful because he can check everything in system and watch what is happening. Did you have a well capacity test done for your well when it was put in service. That would help you understand what is going on as well as doing a demand audit on your water needs. You could just be maxing out your systems capacity for the tank and pump flow. Gotta be one or the other. Aloha

To protect our pump and keeping cost down I bought a new square D 40/60 pressure switch with low pressure cut out. I'm pretty particular about keeping an eye on watering and pump flow, but the wife is not as vigilant. Better to have spent this $30 then to rebuild or buy new pump.
This doesn't fix issue but gives assurance on our investment.
Happy watering.

Jet pumps are very inefficient, you need water being pumped back down the hole (in the small pipe I think)
If you have too many outlets open there is not enough water going back down the well.
At those depths you would probably get more water with a .5 Hp submersible pump as you are with a 2hp jet

You have to set the pressure at the pump and keep it at that pressure

There is a way to set up jet pumps so that they will never "suck air" and loose their prime. If there is like 30 foot of pipe BELOW the jet. As the water level drops, the flow will reduce to small or nothing. This is based on the fact that a suction, or like a shallow well pump, they can not "suck" water over around 30 feet.
Jet pumps can be very efficient when set up correctly. Mike, a proper jet pump has a regulator that controls the amount of water send back down that take priority over the water delivered. Proper setting of this is important.