Mystery Vibration - 74 Cherokee 140

Well, I finally have one. The weird vibration out of nowhere. About 6-7
flights ago, i noticed a very slight rhythmic vibration during a smooth
straight and level flight. About 4-5 seconds of smooth flight and then
2-3 seconds of vibration. I chalked it up to the air conditions. Over
the years, I have felt a lot of different vibes depending on the day.
Each successive flight since then it has become more noticeable. The
pilot wife even noticed it, and she has no mechanical inclinations.

Tried dropping mags off with no effect. Uncowled the beast and the
mounts and accessory brackets all look good. Cold or hot engine has no
effect. It is there from take off to landing in 1 hour plus flights.
Mixture appears to have no effect. I tried running through several tanks
of fuel to see if I got a bad load, no change. Checked the airframe for
any loose parts, none found. We have not done anything recently that
could cause it (replacement, maintenance, adjustment, plug change,
etc.). Plugs all clean, harnesses in good shape.

In bouncy conditions, it comes and goes randomly. Feels a bit like a
very light engine stumble or prop out of balance. It is better or worse
depending on RPM, but it is there pretty much between 2250 and the 2700
red line. Cannot tell at lower rpms, the thing usually was not very
smooth throttled back.

A couple of flights ago, I climbed to smooth air and it was not there,
unless I applied a bit of forward pressure on the yoke. Once back down
in the mild bumps, it was there again. Today, I climbed up to about
5700ft to find smooth air. Running 70% power or a bit less at 2475rpm
and again, it was very smooth without a hint of vibration. I tried to
induce it by pushing forward on the yoke, but this time it had no effect.

I'm going to do a compression check with the wrench. Pulling the prop
through after a flight yields 2 successive cyls with more compression
that the other two, but not a dramatic difference. I know how a 60lb.
and under cylinder feels (all too well unfortunately). Never noticed any
vibration or power difference when we had soft cylinders before. The
thing is making what feels to be full power.

Thoughts?
Thanks,

Mike