Hey there, "My name is not Jessica"
Several things come to mind. For now, I'll assume that your form is really as good as you say it is. My first thought is, "how
consistent is your
toss?" Without a good,
well-placed & consistent toss, the best form on the planet can still yield erratic serves.
Are you
pulling your
head & eyes
down early? Andy Roddick does, but most ppl cannot serve consistently if they do this. Instead, try to emulate Roger Federer. He watches the ball and his contact point on the serve for as long as possible. His head only moves enough to get out of the way of his back shoulder coming over the top (for his shoulder-over-shoulder rotation). His eyes remain trained on the ball and subsequently the contact point even for a short time after the ball has left his strings.
Does your
tossing arm continue upward after the ball release? If you look at most elite servers, that arm is nearly
vertical after the release and stays there for a while -- until the racket head drop (scratch position).
With your tossing arm in this position, use your outstretched hand as a guide, a
spatial reference. This hand should give you a better sense of the ball location with respect to your body and should help you to compensate for minor variations in your toss. It should also be a guide to let you know if you should even swing at a given toss.
This spatial reference should also help you with the
timing of the upward swing of your racket to meet the ball. Try to see when the ball is about 1.5 feet (0.5 meters) above your vertically positioned tossing hand. Try this as your cue to swing. If this seems a little late (i.e., you are not contacting with your racket arm full extended), then swing when the ball is within 2 feet of so of your outstretched hand.
If you do not see this relationship between your tossing hand and the location of the ball just before contact, there is a good chance that you are pulling your eyes & head off the ball and contact point too early.
You should learn the slice serve hit with the continental grip.. Think about swinging alongside the back of the ball rather then trying to hit the back of the ball hard. it sounds crazy but it works..
I'll 2nd this. Actually, any type of spin serve should do -- topspin serve, slice serve, or topspin-slice. Any spin serve that you can get into the box consistently.
.