If your snap rolling out while doing an outside loop, I agree with the first reply. Your CG is likely too far forward. If your snapping out as you say you are (and adding more elevator makes it worse) you are stalling the wing. You can stall the wing at high speed by increasing the angle of attack beond the limits of the aerodynamics of the wing.

Check your CG and your engine thrust angle and give it another go!! You may also want to check the wing as well as the horizontal stab incidence.