Montague's advice is great (as usual!). Follow it. It really does sound as though you're running a little too lean. It may be fine on the ground, but once in the air, the prop unloads a little and the engine leans out some more. Either use the "pinch" test, or hold the nose of the plane up at least 45 degrees at full throttle. You should also hear the rpm increase, then decrease again as you lower the nose to level.

You should also be fine with an OS A-3 glow plug if you can't find a #8 (best), and Montague's prop recommendations are right on too.
Dennis-