I actually had the same problem before the patch was released. Regardless of system specs, this is what I determined....
If you lose the Terran Incognito mission, then your screwed. Subsequently, EVERY TIME you try to load your game from the autosave OR from a manual save JUST BEFORE THE MISSION, then the mission is totally BUGGED.
Fortunately for me, I like to keep multiple save points throughout a game. Therefore I was able to bypass the Bug by going to a previous save point which was one or 2 missions prior to Terran Incognito. Of course, If I failed the mission again, I would have to keep going back two missions until I got it right.
Anyway, it's still early in the morning, so I hope my explaination is easily understood. (My thoughts are fragmented.)
If you have questions on how to get by the bug, feel free to pm me.