Before the Klassic Skins started causing desynch problems, the main cause of desynchronizing errors was due to not using default controls. Yes, this is totally stupid, but its true. Those desynchs would happen in the middle of a match, during combos with heavy button mashing. The Klassic Skin desynch always happens either on the character select screen, or before the match even begins. So my advice would be, if you desynch in the middle of matches, and you use non-default controls, try switching to defaults. Otherwise, there's probably nothing you can do about it.