The crashes in multiplayer are specters..... I have seen missions 1 and 2 run all the way past 6 hours, and seen them crash within an hour. As in other missions as well. My game likes to crash on me when hosting, right as I hit the launch button to fire off a mav after about 2 hours. Mutliplayer is not 100% stable and thats just how it is for the moment :( from what i have seen, any time you have both A10's and KA50's it tends to cause crashes after x playtime.
Did you have KA50's flying with you?
Also, did you have many players that would leave and join a lot?
Also, if you were just letting it run, were you doing other things while it was running int he background?