If it is the cages could it not simply be that the screws are working loose? How about some of that loctite thread stuff (technical term) you know the blue stuff that was probably on there from new, that way when tight they stay tight.
you can eliminate any thoughts of it being the bearings by removing the cages and riding them without
i would give the face of the pedal where the cage sits a roughen up
loctite threads . dab of grease on the underside of the heads . tapered philips ? kitchen scourer the taper/recess of the cage and check the bolts for galling