I agree with biclocal, but it's not just the grip. Even if you change to a strong grip but line up incorrectly then you are still likely to slice the ball. For example, if you look at the target area and think to yourself, 'I'm going to hit it left to right so I'll aim more to the left', bang!, another slice. This is because, you probably aligned your body so that it is pointing left of the target. If the club face is square or right of the target, then you are likely to swing outside to in, causing you too slice the ball.

When you align yourself, pick a marker a few feet in line with the ball, but also in line with the target. This makes it easier to align yourself correctly. Square the club face to the marker, and your body slightly right of the marker (slightly). It will feel unnatural at first as if you are hitting exactly where you feel you shouldn't be hitting the ball. However, this will give you a more consistent swing path, and should prevent the out to in (slice) shot.

Cheers.
Lee.