Hi Jamie,
It is often the case that we seem to be able to hit our driver on the range really well, only to venture out onto the course to find that we can hardly hit the ball. The main reason for this sudden decline often lies in trying to hit the ball too hard, we move away from the natural rhythm and tempo of our swing. The next time you are out on the course and you need to hit a driver try this. Take two practice swings with your 7 iron, then two with your driver trying to recreate the same smoothg rhythm you had with the 7 iron. NOTE: Do this when you are practicing and the course is not busy, so that you do not hold anyone up

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The slice is likely coming from the same problem. When we try and hit the ball too hard are body becomes over-active and our arms and hands cannot keep up, thus creating and out to in swing path, which imparts left to right spin on the ball. Remember: Swing within yourself.
Good Luck & Let me know how you get on.