Cm870 random stumble

[Precisionw900]

My 870 has a random thump out of the exhaust that it will do every so often. It may do it under load or steady throttle. Have one fuel dampener that is seepy so ifsm will be comeing off for a rebuild but while it’s in the shop I would like to figure this out. I don’t believe it’s any air in fuel but will test that first.

My theory is a metering or timing acctuator is randomly hanging from a worn step in a plunger. As just one simple thump and that’s it, no stumble or running rough under load.
I will say that idle isn’t quite as smooth is I would like.
Anyone have similar symptoms?