Okay, the leveling off immediately suggests an incidence problem. Leveling off gradually as the airplane picks up airspeed is, of course, normal. This would suggest to me that you may have insufficient wing or stab incidence. An airplane flying straight and level is a balancing act between the weight of the engine up front, the wing, and the stab. The climbing indicates that the stab is providing too much downforce, which is caused by an excessive negative incidence. Let me guess, the stab incidence was two degrees too far down (negative), right?

The twist of the wing is a different issue altogether. Like your instructor said, it can cause bizarre stall characteristics. These things happen from time to time, it's just a matter of adjusting everything out. This takes some time to do properly. Just remember, make one correction at a time, fly it, and let the airplane give you feedback. The airplane will tell you what's wrong if you can decode its language.