Show871 et all,
One thing people often forget to look for or even think about is lateral pressure on the throttle arm - positive. With the engine running (throttle setting doesn't matter) pushing in on the throttle barrel will force the low speed needle a little closer to its seat. The throttle opening will remain pretty much unchanged but the fuel supply will be less causing it to run lean. (I'm sorry, "seat" may be a little misleading. Actually, I it pushes the low speed needle closer to the highspeed spraybar.)
I had a finicky engine such as this at one point and finally found the answer - the throttle pushrod was pushing laterally in on the throttle arm above half throttle causing errattic operation. I adjusted the throttle pushrod such that there was constant pulling force on the carb/throttle arm/barrel. It may sound strange but it worked. The thin spring sitting compressed behind the low speed needle is there to keep the throttle barrel pushed out. Any amount of force opposing it will defeat its purpose. Check the exploded view of the engine to verify its location.

It will only take a few seconds to check to see if this may be part of the problem.

I agree with the small amount of castor recomendation. I've had excellent results with Red Max (2% castor) and Wildcat Premium Extra. 5-10% nitro. K&B 1L plug.

Glad to hear you got it running well & Good Luck