I think the most significant point is that the engine got steadily better
over an hour of subsequent cruise flight. Four hours later, it's back to
normal. If there had been an underlying cause, it should have gotten worse.

The weather here in the east has been terrible. While those in LA and
similar environs might chuckle at what we call hot and humid, we still adapt
to the conditions. Perhaps some other club members aren't as aggressive
about leaning as they should be and it hasn't been a problem until this long
muggy stretch.
--
Roger Long

The only other things I can think of are dirty fuel, leaking valves, or
perhaps the wrong type of spark plugs was installed.