Try a plug other than the OS F. I would try 15% or more nitro and a cooler plug such as the K&B 4C. But try Fox Miracle plug, Hobby People's Thunderbolt, Enya #3 and #4, and others. Also if you are running it on the bench try differant tank heights, you may have it set too high.

Oh and when you use higher nitro fuels there is a flat spot where there is little change in RPM, set to the lean side of that flat spot and then it may run well at the rich side of the flat spot when in mid range.