If you look at your racket handle it has 8 panels. Count from the top (clockwise) and you will find 8 panels. The top panel is number 1, and so on. You said you have shifted to continental (handshake) grip for serving. In a hand shake grip the base knuckle of the index finger is on panel 3 (the flat right side panel) and this is called Eastern Forehand Grip. No wonder that you are slicing the ball because that's what happens with forehand grip. Generally, those players who have extreme western grips for forehand, when they shift their grip for the serve, think that they are serving with continental grip; but the reality is that they serve with Eastern FH grip!!! Because it is a long way from panel 5 to panel 2 for your hand to move!
The service grip is continental grip (the base knuckle of the index finger is on panel 2 or upper part of panel 2 (the panel between 1 and 3). As you hit you pronate so that strings are lined up with the ball.
What is pronation? When the wrist-forearm rotate anti-clockwise!