OT but may be worth sharing: And something else I once heard ... spark plugs
are subject to vibration, and the ceramic insulation may crack over time,
resulting in carbon tracking and reduction/loss of ignition spark. To sum
up, the story is that plugs may look good, but may in fact not be. So
replacing then every couple of seasons is I'm thinking not a bad idea ... I
think I'll include bi-annual plug replacement in my preventive maintenace.

I think it's more likely something affecting only some cylinders, because
of
the coolness of one manifold while the problem is active, resolving as the
problem disappears.

In my V6, there's a ball/spring mechanism in the cooling system that I come
across every fall when I pull all the water lines for draining/winter
storeage. I've just now looked through my Seloc manual, and don't see it. It
may have something to do with controlling water flow to each manifold ...
dunno ... I'll keep looking for info on that. If one manifold is cold, that
says to me that's where (most of) the water flow is going.

And it could be you have 2 problems. Perhaps you had/have some water in your
fuel/tank ... gas line antifreeze (alcohol) would absorb any water allowing
it to pass through the system. Gas treaments apparently also absorb water.
One reason I picked a carburated engine is that they're much more resistant
to water in the fuel ... may stumble a bit, but the water tends to make it's
way through the carb eventually ... unlike an injected system which I
understand may have to be torn down to get the water out. So, a water
separator is a "shall" for injected systems.