Russel,
it could be many things, either rich or lean.
if this is on profiles my experience is that there is a tendancy for the engine to go leaner in the first couple laps after take off. This is because the tank pickup is outboard of the needle and the head pressure felt at the needle decreases as centrifigal force builds up. It can also be going rich if you sit on the ground for awhile, the engine will heat up and as airflow goes across it in flight it cools so goes rich. It depends a lot on variables. are you coming to Eugene? perhaps we can look at what you have while your there and help ?
the problem with online diagnosis is that there are so many variables, what prop, what fuel, what engine, what tank, what flight speed, what altitude,
and the most important thing is that by listening to what the engine does in flight, you can most times tell if its going rich or lean. As long as you arent setting for a lean two stroke on the ground ( in other words, are you peaking the engine then going a touch richer) then odds are that it is going rich in flight and slowing