Yes, from your description it does sound like an alternator issue. The self test takes place right around 18-20 mph and is in the area of 1 second. All it does is cycle the pump motor but how much current it draws to do this is anybody's guess. If your alt is dipping like that you might want to throw a volt mete on it and check it out while duplicating and be ready to hang a new one. If it comes down to a new alternator do yourself a favor and stuff in a Bosch rebuilt, they are worth the money and let's face it, how many times do you really want to put that alternator in?

Mark