A slipping belt will cause this kind of symptom also. Usually a slipping belt will squeal when starting up and accelerating. Check to make sure belt is not fraid or loose. If ok, then your most likely looking at an alt. But note, that a lot of times when the alt. goes bad, it takes a weak battery with it. most people buy the alt. first, only to find out if not right away, but soon, that they also need a battery. In this case, if you have a fuse it shouldbe good, because the alt. seems to be partially working.
