You slice. Why? If you're like most weekend players, you "roll" your hands to the right as you start the club back. This move feels smooth, but it rotates the clubface wide open, setting you up for a card-killing banana ball.
As you start your backswing, apply downward pressure from your left thumb to the top of the grip. That will keep your hands from over-rotating.
To get the right feel, make practice takeaways while holding a pair of kitchen tongs in your left hand. Set the tongs open at address, and as you start back, close them by pushing down with your left thumb.
Try a few "swings" with both hands on the tongs, then repeat the same feel with a club.
Presto! No more slice.