How do I stop bouncing my knees up and down when I canter?

I ride a beautiful 16-hand Quarter Horse gelding named Montana Sky (Monti). He's really smooth and comfortable, but I seem to have a problem with keeping my knees glued to the saddle. I am fine at the trot, but when I canter my knees slide up and down with every stride, and that causes me to lose my stirrups. Help please!