How are you switching classes? I can't be sure off the top of my head, but I believe that if you change the class, it will not save (meaning $data_actors, $data_classes, etc, etc) will all remain the same. I don't have RMXP open in front of me at the moment (at work), but it may be reading the experience tables off of those variables, which will be wrong, since they are remaining untouched.
BTW: "make_exp_list" is a method. No need for an attr_accessor, which are used for variables.