carb cleaning ?

What engine??

If its the 914, then you are probably leaking boost pressure out of your carb float bowl gaskets!

oops forgot to mention its a 912S.

have a new theory that a manifold air leak could cause strange intermittant fluctuations ? unsure how to check. Surprised it doesnt run bad all the time. 

have problem of nearest Rotax mech is 150 mi away, don't want to remove and dissassemble carbs if thats not the cause.

It is unlikley that  BOTH carbs have EXACTLY the same problem.

If one carb were to have a major problem the engine would be shaking like crazy because the other side would still be making full power.

Your problem is much more likely to be a fuel supply problem (White Plugs) common to both carbs.

Do you have a fuel pressure gauge?  AUX Fuel Pump?  Clean Tank Sumps (Water?)? 

Check the fuel filters. (Limited/Restricted flow)

You made full power for long enough to get it well into the air then ran out of full power but maintained lower power levels.

I would move carb cleaning a few notches down the hit list until other theories are ruled out.

What Bill said. :)  Sounds like fuel supply issue.

Where does your mechanical fuel pump vent line terminate? It must not be exposed to the external slipstream. Where do your individual carburetor vent lines terminate? They must not be exposed to the external slipstream.