LG VX-8100

This is really starting to bug me, when the phone is on Silent or vibrate only it will sometimes ring when people call (at full volume), but it doesn't always do. I would say that it does it about 10% of the time when people call and it is on a no sound mode. Is this just a problem with the firmware? I am going to take it in to get the newest one, but has anyone else herd of this happening or have a solution? It is very frustrating especially during class. Thanks