I'd like to introduce to my campaign a human NPC that suffers from vague and uncontrollable visions of the future.
Are there classes, feats, magic items, or templates that give such an ability?
... would be a good start. I would combine it with a custom flaw: Uncontrollable Ability (select one limited use ability, that ability now triggers randomly (percentage chance or the DM may choose when it activates).
However, having suggested this, I cannot recall any pre-existing material which would exactly match this effect.