For a modern engine like this, 25% oil is probably too much oil. With the tight fit of the parts in todays ABC and AAC engines, they don't need as much oil, and believe it or not, just creats more friction. No room for the oil. And I like to run a 50/50 mix of synthetic/castor in my OS engines. Try SIG Champion fuel in either 5 or 10% nitro. It has 20% total oil content, and you might find it needles better. Also try running a Thunderbolt plug, aGlowDevil R/C long, or a SIG R/C long plug.
I have run rear needle valve set ups for a long time. C/L speed, racing and combat guys have been using them for years also, and they have to have as consitant setting as stunt flyers do. All the dos and don't that apply for standard needle valve installations apply top rear set ups also. The newer rear needle valves do leak a little air past the "O" ring, I think, though. I replaced the one on my LA.25 in my P-Force with the older FP rear needle valve and it works much better.
Make sure you are not over proping the engine also.
Good luck and have fun,
Dan McEntee