I know what you are describing. You are being nicely lifted into the air when the horse bends it's back , reaches under and when it pushes off with the first stride of the canter, it lifts you upward. It even feels as if the horse is going uphill. The next stide kind of carries forward, then the third stride the horse is stetching further outward, reaching out with the inside front (leading leg) and you feel more "impact" or jounce and the general feeling is of the horse goind "downhill". Then it has a moment of suspension while it coils it;s pelvis under itself and gets ready to reach under with the outside rear leg and push off, which again lifts you into the air. Right?
The only bad thing is if you get lifted and stay lifted while the horse goes down (you get "air" between you and the saddle), when you come back down, the horse is coming up and your butt will collide with his back coming up and impact on the saddle/back, before you are lifted again.
It's like the horse has a sine wave and you have a sine wave (remember physics?) and if they are too offset, then you will be interferein with your horse's movement.
If you go up, get air, and stay up, like in two point or a good "half seat", then it can be a very nice way to ride a canter. Very easy on your horse and very light and adaptable for jumping and such. But if you go up and down on your own rythm, not matching the horse's , then that is the worst.
So, either match your up/down movement to the horse as best as you can (needs very loose pelvis and relaxed legs, sit up straight and go WITH horse) OR, ride in half seat or two point and ride "above" the rythm of your horse.
Does that make any sense?