I had a similar problem last year but a 30 minute lesson got it fixed.
If you're driving your left hip towards the target and getting ahead of the ball a good swing thought is to imagine you have a clubhead attached to your right hip and try to make contact with this before the clubhead in your hands.
This will have 3 effects;
1 Encourage you to concentrate on rotating your lower body into the ball rather than sliding ahead of it.
2 Promote correct sequencing of the downswing, preventing your upper body from over-powering your lower body.
3 Promote correct weight transfer sequence
If you slow your swing down until you get this sequence embedded you should see your drives straightening.
I hope this helps
