The short explanation is that the tail surfaces are TOO SMALL! When you add the fact that no two motor/prop combinations provide the exact same thrust you are in for problems. The nose of the plane being effectively BLUNT is acting like a parachute above the CG and so the plane is having its nose pulled upwards with increasing speed.
Your statement about the roll is unclear but with asymmetrical thrust I am surprised that you actually managed to have it fly at all.
Things to do or try!
Mix some rudder, (Differential), in with the motors, (motors on 2 separate Rx channels).
Increase the vertical fin and rudder areas.
Increase the horizontal fin and elevator areas.
Try having the motors spin or rotate in opposite directions to each other.
Ensure that you include a down thrust angle in the motor installations, (start at 3 degrees).
Ensure that the wing incidences are equal and that the motor thrust angles are also equal.
Do your testing slightly nose heavy as the tail area may not be able to handle too much positive loading until it is markedly larger.
Motor positions in line with the wing LE or slightly above will be best as a starting point.
Round off and close in the fuselage nose.
With any change you try note its effectiveness or uselessness so that we can assist further.
Have fun!
