I think you have something else wrong. I'm running a 16x6 Master Airscrew Classic on a Magnum 120. No speed issues at all. I agreee flaps should not be used on take off for a maiden flight, but I have done several with no problems. My plane weighs 12lb empty, fixed gear with a 1400mah battery and the received secured to the rear edge of the servo tray. With no additional weight the CG balanced right in the middle of the recommended range.

If your CG is spot on (with the gear down!), check the horzontal stab incidence on each side to make sure they are the same. It's possible to get them crooked since they install only with the single aluminum tube and no alignment pin. Check your wing for warps. Make sure your flaps are even both extended and retracted. Also check for slop in the flaps.

Sounds like it a motor problem and/or CG issue. With 34mm flap travel my plane will crawl around at or under 1/4 throttle. It is very stable and predictable.