When a plane banks, the wing has to produce more lift to avoid diving. If you bank far enough over, the wing can't keep up, and you dive.

To be able to do a highly-banked turn, you need to get the wing to produce more lift. You can do that two ways. One is to increase the angle of attack. You do that by pulling back on the elevator. This also increases drag, and reduces airspeed, and if you over-do it, you wind up stalling the wing and going down in a hurry.

The other way to get more lift is to get more air moveing over the wing by just flying faster. With heavier models, it's quite possible to fly along straight and level with just enough speed to fly straight, but not enough speed to turn. Lots of models are crashed this way because the pilot slowed down too much when doing downwind before landing. When the pilot trys to turn, there isn't enough airspeed to produce the needed lift, and the plane goes down.

So, you are over-banking for your airspeed. Both suggestions above are right on as well.

Try increasing power before starting your turn, and go easy on the rudder. These things don't react immediatly, you need to be paitent and let the plane resond to your controls.

Oh, if you play a lot of video games, you might be in the habbit of pulsing the controls, like pushing a button over and over. That doesn't work very well. Try a small stick movement held for a longer period of time, and think slow, smooth control imputs, nothing sudden or jerky.