Lyc IO-540 won't make RPM.

Do you ever have those days where you're just scratching your head?

I've got an IO-540 (strapped to the right side of an E-Model Piper
Aztec) that won't generate full RPM. Let me describe the takeoff
sequence.

During the run-up, both the feather check and the mag check are normal
(done at 1700 RPM and 2000 RPM respectively.) During the takeoff roll,
all seems normal, but initial climb is sluggish (not alarmingly so, but
just enough to notice). The right side RPM indicator shows about 2450
RPM, while the left side is showing just above 2600 RPM (as usual).

When setting climb power, bringing the power back to 25-inches on both
sides will not cause the left side to lose measurable RPM, but the
right side loses about 100. Synchronizing the propellers does cause the
"beating" to go away when the RPM indicators indicate the same RPM, so
I'm confident that the RPM gauge is reading correctly.

During the initial tear down, it was noticed that the set-screw for the
right governor was set at about as high an RPM as possible. So the
initial feeling was that the governor was having a bad day.

So far, we've:
1. Sent he governor out for overhaul. (Prop shop reported that it was
fine.)
2. Swapped the propellers between the engines. (Problem does not follow
the propeller.)
3. Swapped the governors between the engines. (Problem does not follow
the governor.)
4. Verified full travel on the propeller controls (both in the cockpit
and at the governor).

The tech reps are suggesting that we send the engine back to Mattituck
(it's a factory remanufactured engine which has been "Mattitucked" with
about 100 hours on it), but I get the feeling that the tech reps and
mechanics are essentially in guessing mode.

Does anyone have any suggestions on what else to think about before we
pull the engine?

Thanks!

-Rob