Pressure regulator problem?

My water pressure in the street is 125 psi. I have a Wilkins 600 pressure regulator on 1 1/4 copper pipe.
I don't know if I have a problem with the water pressure regulator for the house, which is set for 50 #: Usually, the static pressure is 50#.
Sometimes the pressure goes up to 80 PSI but returns to 50 # when I open a faucet. I wonder if this is normal and may be caused by thermal expansion if the water heaters fire up. Is there anything I should or could do about this?
When I open several faucets inside the house, the water pressure drops to as low as 30 #.
Sometimes, when I open a faucet, the water comes out at low pressure (maybe 30 lbs) for maybe 10 seconds and than gushes away at the more normal pressure of 50 lbs. The difference is quite noticable.
Is the pressure regulator defective and should it be replaced? A new regulator costs $ 300 (installed). I don't mind the $ 300, but I would hate to replace a perfectly good regulator.
Is it worthwhile to recondition/repair the pressure regulator?
The Happy Iconoclast
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