Well, my first thought was maybe you put the old belt on backward (i.e. opposite the original rotation) and that might account for it falling off, but you said you also tried a new belt so it couldn't be that. The old belt could have a bias as it streched and wore, but not a new one.
You did check to make sure all of the v groves match on the belts and piullys I assume. I'm sure you did, but jus tthought to mention it. Other than that, perhaps it is no longer aligned or the bolts have started to vibrate loose. If that is the case, here's athread that expalins (basically) how to remove and repalce the power steering pump. Maybe this will give you an idea on how to check the attachment bolts and see if that is a problem or not.
http://986forum.com/forums/general-discussions/29323-power-steering-pump-removal-installation.html