Strange engine event

For the last 200 hours I've recorded all the data from my engine
analyzer which makes it easy to look at such things. Anyway a week
ago I was flying along steady at 12 thousand feet when I noticed
an unusual shift in the engine bar graph display. Looking at the
recorded data I see that all flight data is rock steady and all
engine data (and fuel flow) is also rock steady except for EGT1
and CHT1. From the beginning of the event this is what it looked
like:

Time after event EGT1 CHT1
---------------- --------- ---------
t = 0 1355F 300F
t + 20 seconds 77F rise 7F drop
t + 80 seconds 100F rise 19F drop
t + 3 minutes 114F rise 25F drop

After that it was pretty much steady, but I reduced power after
a few more minutes. (By the way, this is a Turbonormalized IO550.)
So EGT1 which was the coolest (as usual), rose to the level of the
warmest few cylinders. And CHT1 was the coolest (as usual) and
dropped away even more below the average. Still the final temps do
not seem that alarming, but when you look at the graph it looks
really strange since I've never seen such step changes unless it
was in response to some control input.

In the few flights I have taken since that event, I haven't seen
anything unusual like this happen again.

So to those of you who understand internal combustion engines -
what can cause a sudden drop in power in one cylinder like this?
Could something have fallen into this cylinder or could a
valve stick for a while or something? Or is this a sign that
something is breaking? What would you do about it? -
- Keep a close eye on it?
- Do an oil analysis?
- Ground it?
- Have a mechanic look at it?

Can a bad mag or spark plug cause such a thing? (BTW the engine
is a factory remain with 300 hours on it).

Thanks in advance for any insights you may have.
~Paul