This may be a shot in the dark, but many years ago we had a similar problem
(slight sag in power during initial climb) with the O-320 in a Cherokee-140.
After some troubleshooting we determined that most of the lost power could
be restored by significantly leaning the mixture. That led us to determine
that the cause was a partial obstruction of the intake air path between the
air filter and the carburetor, causing the mixture to be excessively rich.
I don't recall what the obstruction actually was (or how it got in there)
but it seems that it would be most pronounced at high power and high nose-up
attitude.

If you haven't done this already, next time the "sag" occurs, see if leaning
helps a bit. Sorry if you have previously reported that you've already
tried this.

--
-Elliott Drucker