engine startup problem

Hi -

Very rarely, one of the 6-cylinder Lycoming IO-540's in my Aztec acts
up after startup. There is a fair amount of vibration, and a high MP
(20") for the resting 1000rpm, and if the engine monitor is turned on,
it shows three of the six cylinder (1, 3, 6) not making any heat.
Turning on the electric boost pump, playing with the magnetos,
mixture, throttle, all seem to not help.

After a shutdown and a minute's rest, a subsequent startup is usually
perfect, with all cylinders firing well throughout the runup and
flight. Post-flight visual inspection of the cylinders appears
normal.

Does this set of signs ring a bell toward a possible explanation?

- FChE