You might want to temper the chocolate before using it. Here are directions and an explanation from Food TV:
"Definition: A technique by which chocolate is stabilized through a melting-and-cooling process, thereby making it more malleable and glossy. Commerically available chocolate is already tempered, but this condition changes when chocolate is melted. Tempering chocolate isn't necessary for most recipes, but is often done when the chocolate will be used for candymaking or decorations. Chocolate must be tempered because it contains cocoa butter, a fat that may form crystals after chocolate is melted and cooled. If these crystals aren't stabilized through tempering, they can form dull gray streaks called bloom. The classic tempering method is to melt chocolate until it reaches a temperature of 115 degrees F. Two-thirds of the melted chocolate is then spread on a marble slab and worked back and forth with a metal spatula until it becomes thick and reaches a temperature of about 80 degrees F. This thickened chocolate is then transferred back into the remaining one-third melted chocolate and reheated to about 89 degrees F. for semisweet chocolate, about 85 degrees F for milk or white chocolate. The quick tempering method is to melt two-thirds of the chocolate to be tempered to a temperature of 115 degrees F then add the remaining one-third (finely chopped) chocolate to the melted mixture, stirring until the mixture has reached 89 degrees F and is smooth."
This works very well and insures a beautiful product.
Also, mickeyboat, I agree that you are heating your chocolate too much. The melting point of chocolate is between 104 and 113 degrees, about the temp of hot tap water, so you need to go slowly. For example, I just melted 2 oz of unsweetened choc (for chocolate thumbprints..mmmm!) and in a large microwave, using a glass measuring cup, I started with 40 secs, stirred and then went another 30 sec. When I took the choc out of the microwave, it appeared to be not fully melted, but after I gave it a bit more of a stir, it was perfect. Remember, better underdone than overdone with chocolate!