When sailing in about 20 knots of wind my boat which is cutter rigged tends to head up stall and then fall off can you tell me why this is?

Sound like you are cutting too close to the wind. Try backing away from the tightest point by two or three degrees. But if it seems to be automatically heading too close in, your rudder may not be as secure as it should be. In other Words, it could be drifting while your tiller/wheel is steady.