So what do you tend to use before it happens?
Hmm... Well, the no AP cost movement and being unable to do anything generally happens just as it's becoming my turn, and if there is any trend for me, it would seem that it happens right when I'm using Armor of Frost. No single ability have I whiffed more than that one.
As for my character locking up, and not being able to do anything... It happened once just after the end of the croc fight, and it happened once just as I got done buying stuff from Rezzik. That's all I can remember.
Also worth noting, is that this only happens to me during multiplayer.