If you're not adding water-based coloring or flavoring, then the problem probably IS the heat. The stuff stiffens up pretty easily, with either high heat or sustained heat. I try to melt just a little at a time, and keep adding new chocolat to the mix as I go along. Seems to help keep the texture smooth.
The microwave is fine for candy, but it can get too hot there too. I use 1/2 power and stir every 30 seconds after the first minute (unless it's a very small amount of candy then both times are shorter).
For shortening you should add a little at a time until it's the consistency you want.
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