If your plane stalls and snaps out of the top of a loop, or any other high-g maneuver, you are using too much elevator for the weight and wing area of the plane.

When you first start a loop, for example, your plane due to weight or amount of elevator, is close to a stall. As it slows down, it drops below the stall speed so it snaps. This usually occurs at the slowest point, the top of the loop. On a split-S, you probably pull up first, or you pull harder since you are going downward, again causing the plane to drop below stall speed.

Basically, you are trying to get more that your plane can give. You can reduce the amount of elevator and/or add more power without adding weight or maybe add flaperons.

Here's how I would reduce the elevator. Fly straight and level and do a full up loop. If the plane snaps out, land and reduce the elevator movement. Take off and try it again. Keep doing this until you can do a loop using full up without snapping. Use this amount of up elevator for your normal flying. You should do the same thing using down elevator.