From all of the things that I have read on this - it appears that there should be 2 spawns of these on a 1 hour spawn basis - with the possibility of the named ( which I have seen). After spending about 2ish hours in here I have only seen 1 heretic invader spawning.
http://everquest.allakhazam.com/db/npc.html?id=10356
the comments below suggest that there are two spawns for this.
Maybe I went to the bathroom right at the spawn time of the other one, but I have been quite actively looking for the other spawn.