Two new water pressure regulators. Won’t hold pressure

I Replaced/changed out pressure regulator 3 times. Flushed out any possible debris. Tried setting at 50 and 60psi. After turning water on then off, it continues to be erratic, sometimes keeping correct pressure & sometimes randomly increasing pressure slowly, to 80 or 100 or at times up to 120 psi Have 2 pressure gauges. Have put them on an outside spigot & inside on washing machine Regulator is located in an irrigation box at least 12”under the ground next to garage (not at all easy to get at) Outside temperatures are mild & not an issue. I’m stumped. Something else must be going on here. Any ideas?