For starters, you didn't state if your using the stock needle valve. If in the event you are, go for the OS 1a needle and have it remotely located on the plane. As your probably aware of this a lighter pitch prop equates to higher rpm's and easier needling. You can adapt a piece of tubing into the stock venturi to hook your fuel feed line onto. I generally use a wd-40 straw bent into a 90 deg elbow.  I don't have a Norvel .25 or a .40 but have used every other engine they make and I enjoy them. A 8x4 apc would be a very good choice prop for the .25. Any air leaks are going to cause inconsistent runs. I can't see the need for 15% fuel. It isn't going to harm anything but 5-10%  nitro would be sufficient for speed limit. In the event your using the incorrect temp range of plug, I can see a problem there. A hotter plug using the lower nitro fuels would be in order. I can say this, I've been flying speed limit for the past year and what worked for me in stunt hasn't been the case in combat. The Thunderbolt r/c long was my choice in stunt and it doesn't stand up well in combat as the filament is very weak and fragile. I believe the best plug I found to use in my Norvel AME .15 was the Mccoy brand. The OS medium heat range I believe #4 would also be a good choice.  Your symptoms sound as if the plug is cooling off. Ken

Those are the same symptoms I had with running car engines designed for hi nitro fuel on 10% racing fuel.  Needle way too sensitive.  It's a compression ratio problem. The hi-nitro takes, I think (I'd check with the speed guys), lower compression ratio.  Takes experimentation with head shims.