rough engine just after power reduction

Last night, on a short hop back to home base after an
instrument lesson (read: extended low power ops),
engine shuddered and ran very rough just after level-off
and power reduction. Run-up and takeoff were normal,
and as usual I leaned like heck for taxi and during
runup. About 80 degrees with blissfully low humidity,
carb ice unlikely esp. in our installation, not very
carb ice prone.

Ground ran with brutal leaning afterwards and checked
all chts/egts on both, r and l mag. r mag ran rougher,
with higher EGT on #4 cyl.

Suggestions? Going out to clean plugs and make sure
all 4 cyl have compression this afternoon, what else
to check? I'm hoping for a badly fouled plug, but
would that produce high EGT on 1 mag?

BTW I envy the chap in the other thread who posted about
"continuing as normal as possible" because an engine w/
a blown valve will continue to make normal power for
a long time. Personally when my engine is running rough
I'm not capable of such savoire-faire, I operate on the
assumption that I really don't know what's up and the
b**ch may quit on me any second. I want as much energy
as I can bank until I have the runway made.

Cheers,
Sydney