Maws show that problem on my WinXP PC occasionally, but also regular NPCs. It's very noticable when it's the NPC you need, so I don't know how often it happens to any NPC I have no business with (Like, you ran out of Forbidden Keys and the Cruor Prospector doesn't load. Or a quest NPC for a quest you've repeated for an hour is suddenly not there.) Walking out of sight range always solved it, but it's annoying.