Rich rote: ".....I do play more front to back across the theremin at an angle to the right, but not directly at the pitch antenna (I find it's much harder to play directly at the pitch antenna).
By playing "on the bias" you desensitize the response of your instrument, and you interfere with your ability to articulate accurately with your pitch hand and fingers. This might account for why you have a tendency to "skate" (i.e. sliding between notes, rather than stepping).
Also, you smother your vibrato because there is a great deal of wasted energy and motion in this kind of technique. Remember, it is PROXIMITY to the vertical antenna that controls pitch. If you're playing on the bias, where your aim is not directly toward and away from the antenna, your movements are going to have to be exaggerated in order to have an effect on the sound, This is not an efficient way to play.
If you are serious about the instrument, you may have to re-think your approach. Are you ready for that?
If you're just havin' fun, then GO FOR IT.