It turns out that the problems with the server freezing on some of our missions are not restricted to just us, but quite common across the ArmA community at the moment.
It's likely to be a mod conflict, probably between Alive / Ace / RHS (and then only on A3 v1.54+), although it hasn't been narrowed down yet and may yet be much more complex than just that.
We run these mods and some missions are completely fine, while others fall over somewhere between a few minutes to a few hours of play.
http://alivemod.com/forum/1552-server-f ... ve-and-aceUntil some resolution is found we'll just have to stick with the missions that we know are stable. This still gives us a good number to rotate, but obviously we'd like to reinstate all of them and add new ones to the schedule that have been created recently.