Question
Matt, Thank you for your time.   Moved into a house on a river and trying to
revamp an existing system.  Moving from problem to problem but this one
has me stumped.  I am pulling water up the bank 15 ft to the 11/2hp jet
pump.  From there 15 ft higher to top sprinklers.  Pump works great for a
while, loses pressure, then picks up again.  You can hear the pump picking up
flow, seems to throw out some air, and the cycle repeats.  Draw line is 1 1/2"
poly- feeder line is 1 1/2" pvc directly to 3/4" hose bibs and 5/8" hoses.
Thanks again.

Answer
Thanks for your question.
First thing I would suggest is check the suction line for air leaks. Try pressurizing the system with a hose from the house. Then go check the pump, pipe & fittings. Even if it is just a tiny water drip, it will be a huge air leak.
Next would be watch the filter while the pump is running. River's tend to have problems with contaminants getting on the filter. While the pump is drawing water it sucks the debris and accumulates on the filter. When the pump stops running all that junk flows away making it look as if the filter is fine.
My guess is you have a combination of the 2 problems.
Good luck, if you need additional help let me know.

Matt