type 4 dropping a valve?

[Farmer]

Better than tearing it apart (!)
Often the smaller problems are the most difficult to find out.

About a year ago, I was haunted one full afternoon by a new plug that suddenly decided to go south on me. It worked in the morning. Then after lunch when I was supposed to do a trim, it didnt work anymore. Naturally the plugs was one of the last things I cecked, right.