Your voltage is too high, or you have hot spots.
Make sure you're verifying the resistance of your coil with a multimeter before putting power to it. Wraps to gauge is only a rough guideline, a starting point; when your coil is done you have to know what the resistance is so you know the proper voltage to put to it.
If your resistance/power ratio is right, then you haven't been working the hot spots out of your
coils. Read
through the rebuildables forum for tips and tricks on that. There are also Youtube vids to guide you through working these out of your setup.