"Roger Long" om wrote in message
.. .
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.

Not true in general. For example, valve sticking will manifest itself when
the engine is first started which will clear up as things warm up. This doesn't
mean all is well with the engine however, eventually you may have a catastrophic
failure.