Its usually easier to go down to 20% nitro in my opinion. Plenty guys like to mess with nitro percentage, but problems like these occur. This problem, may not be necessarily based on nitro %, but I find that most people don't know how to correctly go about using higher percentage fuel. 25 is good, but too many people try 30+ in my opinion, b/c few have the knowledge that initialD has.
If it is humid where you race, I would suggest the #7, otherwise a six would be better. But realize, that this will raise your engines temps. Different fuels and plugs can change an engines temp by 20 degrees (farenheit, sorry
Your high end needle setting was probably set to lean because your low end needle was a shade on the rich side, so this would compensate for the rich low end by giving more power in the lower rpm range. However, when going just before full throttle, that is when fuel demands are at its highest, so it makes obvious sense that your cutting out during heavy acceleration.
If it were your low end needle settings, then you wuold see your engine cutting out at almost any point in the rpm range. This is because a lean low end needle setting gradually overheats an engine anywhere after the three minute mark in a race generally, depending on how lean your carb is.
Just remember that if your settings were your problem, then it is quite possible your plug, fuel, head shim combo caused you to resort ot these settings. However, a problem like this could be many different things and try what you think is best.