It's a common, though nonetheless irritating habit of these cars to stumble for an instant when the a/c compressor kicks on. You'll notice it happen every 20 or 30 seconds while you're driving with the a/c on. As the saying goes, "they all do that."
If, however, you're feeling anything more than one, quick little drop in power every 20 or 30 seconds, that's not normal.
