guys, thanks for the replies

so it seems there's no way I can get this a/c to flat spin as it is.
I might build a larger rudder and elevator. Or first i'll try to tape cardoard to enlarge the control surfaces, then if it proves better i'll build larger surfaces.

frcac - my rudder servo is graupner (JR) C5007, which is a $20 servo, just a bit stronger than bottom end of the market. I run it on 5 cells NiMh. I can try to find more info of the servo (torque etc...) if you need. from what I see it's a bit bigger and heavier than my 3001s

also, what CG location did you retain at the end?