I individuated the reason of the sometimes flippering of custom made animations ....(no it is not the problem of the last frame of a loop to be the same of the first frame of a loop)

sometimes when working in max and u make animation , u may make some corrections like deleting some stuff you have already changed , this may cause a sort of shadow memory effect in the max animation that when exported is recognized by the game and not handled properly so that it makes appear a sort of memory frame that kept the position of the once made change .... there is only one way I have found to resolve this ....

Redo the loop by beginning and use always the back button to make corrections instead of deleting them ....

This is what I have don and seems was the only way to fix the problem .....