I had the same problem a while back...it tured out that i was standing too far from the ball at address when using the driver. I was setting myself up to smash a big one instead of just addressing normally and swinging at it.
by standing too far from ball at address and not standing upright, i found i was swinging outside the 'line' swing path stretching for the ball and then on my downswing was casting the club out, 'throwing it out' and as a result swinging through the ball slightly from outside to inside and slicing the ball. a sign of this was a slight divot line coming to the inside instead of straight.
also sometimes by over swinging, your shoulder turn on the downswing is ahead of your hands and you cast the club out to catch up and swing across the ball.
these are just some observations on my own swing and some things that i have been working on. maybe this will give you something to have a think about.
good luck