What is the brand of gauge? Usually a problem is low pressure, either from a cheap gauge or faulty line (kinked if copper). I have never had a oil pump spring get stronger and increase pressure but maybe something is stuck in the pump making it "stiffer" and increasing the pressure. Otherwise I suggest a 2nd gauge to confirm what the other gauge reads.
I had the same thing happen to my '68 F100, it was the constant voltage regulator located behind the instrument cluster. Switched it out for a new one and that was my problem solved. Worth a try.