skribnod,
You are definitely experiencing tip stalls due to low powered take offs. The 72 four stroke WILL create a lot more torque than that 40 two stroke, and the effect at low airspeeds is much more pronounced.

You mentioned that next time you'll add full throttle just after it takes off; that's the worst thing you can do! That will maximize the torque effect at the lowest airspeed and almost positively result in a torque roll (stall) into the ground. You want that throttle applied BEFORE the plane leaves the ground, then be ready to apply right rudder not aileron, if it starts to roll left.

Hope you have better luck with your new one.
Dennis-