Disappearing NPCs

Weirdest thing ever. Normally I don't ever ask for help on anything but this has me stumped. At times when certain characters login, they're unable to view anything around them as far as NPCs go, however if they walk to persay another section of the map those npcs appear. And if they walk back, the ones are there that weren't before. Anyone ever heard of this? I suspect there's a possibility that for some odd reason it may be guild related. I've tried a number of things. Removing the player's guild seems to effectively stop it. I can't directly figure out what's causing the problem though.