Thanks for the very complete reply.

That's the way I operate the engine. By "Aggressive" leaning, I mean doing
just what you describe as opposed to just leaning enough to see an uptick on
the EGT or start of RPM rise. I can't go LOP with my carburated O-320
without it getting rough. I know "Aggressive" means something more with
Gami injectors, engine monitor, etc. I do "Unaggressive" leaning when I'm
busy, climbing above 2000 feet, etc., and just want to take the edge off the
Lycoming's tendency to foul plugs.

I did lean enough for the engine to get rough. Walter Atkinson said over in
the CPA forum that there is no physical way what I observed could have
happened unless the gauge stuck. Why it came unstuck as soon as we reduced
power a bit is just one of those mysteries. I'll have to keep an eye on it.

--
Roger Long