Well pump turns on every 30 min. HELP

I installed a new Red Jacket pump 5 months ago. My well is 330' deep and the water level is 60'. Pressure switch is set for 30 to 50lbs and the tank is holding a precharge of 28lbs. The system has only 1 check valve that's in the pump.
The pressure will drop in about 30 min. and the pump cycles back on. I shut the water off to the house but the pump still cycles every 30 min.
Could a leaking pitless adapter O ring be causing this? Should I install another check valve? If so where is the best place for the check valve?
Thanks

Yes the pitless could be causing this, so could a pinhole in your droppipe or a dirty checkvalve.

No you shouldn't add another check valve.

bob...

You could isolate portions of the feed line from the well temporarily [using check valves or existing shut off valves] to determine better the location of the leak before pulling it out. Might be above ground.