I have nothing personal against synthetic saddles, but I guess I wouldn't put the word "good" and "synthetic saddle" in the same sentence.
You need to find the proper TREE that is going to fit the shape of your horse's back, and it is no easy task. Typically, on a high-withered horse you are going to need a narrower gullet (but of course that's not always the case) so that it clears the withers and sits evenly on his back.
Also, are you sure you are placing your saddle back far enough? Often saddles will move to where "they want to fit". If your saddle is sliding back during the ride, maybe you need to saddle back a bit farther. A picture would be nice to see how your saddle currently fits your horse.