Oh okay, yes that sounds like something different. You are describing an inconsistent setpoint or a drifting setpoint. That could be a result of being near the limits of the Belleville stack as previously stated, or it could be something that is causing the spring rate to vary each time the regulator cycles open and closed. At least one design iteration of the FX regulator had a lot of slop between the piston's OD and the ID of the Bellevilles. That would allow the washers to slide laterally relative to their neighbors, thus the spring rate may vary from time to time. It's hard to say without some trial and error experimentation.
With that said, for a variation of only 10 bar, it should be possible to tune the gun to be agnostic of it. An unregulated PCP that is properly adjusted can hold a 1 - 2% ES over a range of 20 - 30 bar, sometimes more. A regulated gun that varies by 10 bar is technically no different from an unregulated gun that is operating over a pressure range of, say, 150 -> 140 bar.
By the way you stated again that if the problem were creeping that it would be easy to fix by cleaning and some O-rings. That's almost never the case and I fear that it may mislead others who are reading along.
At first my rifle did experience creep it would raise the pressure while it sat over night. I contacted FX and was told it could be the piston oring, and to check that and clean it. They also sent me a new piston but the old one was still good and exactly the same. And after cleaning the creep stopped. So in my case cleaning was all it took. I just don’t like the plastic piston, I think that has something to do with it. Maybe the washers are sticking in it I’ll have to see if there is any marks on it when I remove it.
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