my best advice to you, would be to slow it down.

The reason you're falling off balance throwing a combo is most probably a technique fault. If you practice throwing that combo reeeaaal slow over and over again, you'll start to feel at what point you're starting to lose balance.

Once you can throw those combos reall slow flawlessly, start speeding it up. As soon as you feel yourself losing balance, slow it down again and rebuild yourself.

.. thats how you can figure out where you're going wrong, but essentially I believe that if you're falling off balance while throwing a combo, you're doing something technically wrong.

Do you have a boxing coach? how long have you been boxing?

.. if you dont have a coach, get to a gym asap, before you start building bad habits that are hard to shake off once they're ingrained into you. Now the hard part sometimes is finding the right coach.

good luck