Well to me it looks like you are doing everything that you need to be doming the only thing I would sugest is that you write down everything you do to keep track of progress.
Barbell squats (this is a breathing/power/explosive exercise) the Stair master. and the cross trainer (elliptical movement) can all build endurance, but 15 minutes of totally intense training of this type is all you need. Work the heavy bag and skip rope for 4-5 rounds during your workouts will give you what you need. Weight training bad for the boxer is a myth. 2 x 40 minute resistance workouts per week are a great way to supplement your regular 2-3 x a week boxing workouts. Best time is shortly after your workout and perhaps once on an off-day. Remember, muscle grows when you are resting, so don't overtrainGoldenBoy said:Sprints Sprints Sprints!!!!! If you want to gain speed and power, trying performing an activity that actually requires you to be as fast and strong as possible.
While swimming is an amazing way to get a nice workout, a constant pace lap after lap will do little to help you in the ring...
Remember, amateur boxing requires only 6-8 minutes of actual activity seperated by 1 minute of rest per round. Throwing a 6 punch combination, then moving out of the way then throwing another 6 punch combination is all so common. You wouldnt just punch a heavy bag in a slow timely manner like robot would you? So dont train in a way you wouldnt fight.