It's a fouled plug, burned valve or broken ring.

Fouled plug, maybe as it could clean itself up, a burned valve or broken ring
wouldn't fix themselves.

I certainly doubt something like a broken ring as well since the
problem has not yet re-appeared.

As far as the fouled plug theory - could a plug really get fouled
that quickly? Remember the plot looked like a sharp step function.
Most of the EGT/CHT change occurred in the first 20 seconds of the event.

So for now I think the injector theory, or the stuck valve or lifter
theory is more likely.

By the way I flew again yesterday for 2.5 hours and the problem
still has not repeated. So it may be awhile (if ever) before we
figure out the real cause.

~Paul