The two anonymous posters at the top pretty much explained the problem and the solution. I don't think it's lag per se, but more like your computer sending / server processing packets out of order. If you stop and immediately fire, the server occasionally processes this as fire then stop, which gives you the interruption. The simple solution is to stop, count to 1, then fire. You can actually move around for a step or two during your shot and still not have it interrupted, try it out sometime. I haven't had a single interrupted shot since I realized what was going on.
Ras <Lost Anarchy>
Human Priest, Ner'Zhul