Never seen the source code for StealthQuake, but I can explain how a similar crash can happen in vanilla Quake and that might point you in the right direction. Take a look at HuntTarget

void() HuntTarget =
{
self.goalentity = self.enemy;
self.think = self.th_run;
self.ideal_yaw = vectoyaw(self.enemy.origin - self.origin);
self.nextthink = time + 0.1;
SUB_AttackFinished (1); // wait a while before first attack
};

This function runs when a monster spots a player, and the dangerous line is self.think = self.th_run. The idea is th_run contains a think function bottled up for future use, and HuntTarget finally uncorks it. But if th_run is empty, then you've just scheduled an empty function to run in 0.1 seconds, and a crash is imminent.

Have a look at your code to see if the same transfer from th_run to think is happening when a monster wakes up, or if a similar transfer is happening from a different field. Then make sure that when you create your monster you populate that field with the correct ai function.