Sounds like you are OK with the prop, the servo, and the linkage, as long as it is all working properly. The YS 110 and a 14x10 prop should give you all the power and speed you need - I had that combo for a bit and was satisfied.

Maybe one thing to check is your rudder throw rate - too much throw would cause a stall, or maybe some other odd behavior. It doesn't take too much rudder with that plane to hold a KE.

Keep us posted.