Lynn, It could be a head gasket but truthfully your information is way to spotty to make a diagnosis based on it. The only distinct impression that I come away with is that your "Gas sta guy" knows just enough to be dangerous and that is it. If you overheated it a couple of times that could be enough to stress the head gasket to the point that it starts leaking. Are you loosing coolant? Often times a head gasket can leak but not cause an overheat condition. Usually it is only when the leak progresses to the point that you are loosing coolant that you will run into overheat problems. In the early stages of a head gasket leak it will usually leak when it is cold and then once the engine warms up it will seal up and quit leaking and run fine. As far as the alternator goes it sounds like he took out your good Bosch alternator that probably only needed a voltage regulator/brush holder assembly and replaced it with an aftermarket piece of crap that went a whole 6 months before it quit working. You cannot expect a pump jockey to know these things and with that I would seek out the services of a COMPETENT Volvo repair techinician if I were you. If you'd like you can e-mail me with your general location and I can see if one of my parts vendors has a trustworthy account in your area that specializes in Volvos, they usually know somebody in most of the major areas across the states.

Mark