First, do you using MAX version 8,9 or earlier? Because Max have some problems with exporting in X file in versions 6,7. Sometimes is all ok, but sometimes is not. But the common problem is the left-hand axis option in Panda x-porter. To correct this, after your model is finished (before rigging), try to Reset X form. To do this, click on Utility tab, and in the options bellow you'll have ResetX form button. With the model selected just click on it, and then convert it to editable mesh. This is kinda strange solution
, but it helps me many times.
Good luck