Not sure what you should do, exactly, but perhaps you can go to a few dentists, and get their opinions as to why this is happening... ?
seems like there may be an issue with your bite (the alignment of the bridge) if enough force is focused on one spot that isn't "right" there goes the foundation...

Had that once with a cap. what a pita.
You could perhaps go to a few places and tell them that you were thinking about switching dentists... (they may give you a free checkup...) then say, oh yeah, my teeth happen to fall out all the time...