It sounds like the pressure is fluctuating but you won’t know for sure until you put a gauge on the system. You will also see if any of the flow devices are closing. I have several analog water column gauges but I still prefer to use a piece of clear Plastic tubing, at least 3/8” ID, and make my own water manometer… The gold standard for accuracy.
I use a 25-year-old Marshall regulator and I move it from RV to RV, because it’s such a high quality.
The newer ones are built super cheap so I wouldn’t try to scimpon one of them.
I set the regulator to 11 inches at no load and I like to see at least 9” when everything is at high fire. Craig