Very likely this is caused by the same "invisbile creature" bug as the monsters that are sometimes unseen, but with highlighting with the Tab button on your keyboard you can see it's nametag hovering around.
I have witnessed this as well on NPC's, on a merchant or two on Mistone between long periods of time. Normally changing areas a couple of times fixes this, if not, relogging fixes the problem.
Most likely a Bioware Aurora Engine bug, so nothing can be done sadly enough.
