Pawnee problems

Our club has a Pawnee with an 0-540 that has been giving us fits for the
last year or so. We are having intermittent problems with power surges that
can be significant and are sometimes even noticeable from the ground. The
problem will occur one day on one tow, and then sometimes disappear for
several weekends. One tow pilot seems to think that the problem seems to
appear any time the airplane experiences noticeable negative Gs.

In the past year or so we have replaced or rebuilt the following:

Carburetor
Ignition harness
Fuel shut off valve
Exhaust system
Mags
Spark plugs

We have also cleaned the gascolator, checked the fuel tank for obstructions,
and checked the vent system.

The engine has about 1300 SMOH and early last season we did have a stuck
valve in one of the cylinders. We switched from auto fuel to 100LL at the
beginning of last season and my recollection is that the problem might have
started or gotten worse shortly afterwards.

I am thinking that the engine is probably due for a top overhaul, but the
problem doesn't sound like sticky valves because some of the other tow
pilots say that the engine runs smoothly when the larger power losses occur.
I have never experienced one of the major losses myself, but I have had a
few of the 100 rpm drops which cleared up after about 15 seconds or so.
When it happened to me I did notice a very slight engine roughness which
does sound like it could be a stuck valve. It also makes sense that if it
is sticky valves that the problem would get worse running on 100LL.

Any comments or suggestions on what to do next? Two mechanics have looked
at it and have not been able to resolve the problem as yet, and we need our
Pawnee back on the flight line. We are close to doing the top overhaul as a
next step.

Thanks,
Bruce