Lost pressure in shallow well system

Hi,
I have a 40 year old Myers 1/2 HP HC460 shallow well pump system in my basement that I use for a sprinkler system only.
Looks like I have 1 1/4 pipes coming from a 4-5" cast iron casing. Lived in house for 30 years with no problems until this spring. Pump used to deliver about 42 psi at pump. It wouldn't shut off at old settings so I had to adjust down to 36 psi. Now when sprinklers are on, pumb delivers a steady 34 psi which isn't enough to lift all all the pop up sprinkler heads in a zone. Water table levels are not an issue. Storage tank is 40 ga. galvanized, no bladder. It has a valve and tube arrangement half way up the tank I think is there to prevent water logging. Any ideas of why I lost pressure?
Thanks

Your jet nozzle is clogged up.

Thanks for your reply. Can it be cleaned or must it be replaced. Is this difficult to do yourself..I'm pretty handy but never worked on
a pump?

It can be cleaned but you will need new gaskets before you start the job and no silicone won't work. It might be difficult to find parts for the old Myers too.
You can always change the whole pump.