The flat latex wall paint that was applied over the glue by the previous owner DID NOT seal the glue. Flat latex wall paint is porous; put water on your wall and watch it soak in. Therfore, when applying another coat of flat latex wall paint, it acts to rewet the wallpaper adhesive and turns it back into a paste. The topcoat of paint is now "floating" on the paste and as you roll back and forth over it, loosens it, picks it up, and/or smears it around. As others have suggested, priming over this with an oil-based or shellac based primer indeed appears that it will seal over the glue and keep newly applied latex paint from disturbing the glue. But, if you've alraedy got aligatoring and a splotchy surface, sealing over this will lead to skim coating the wall to finally correct the problem and get the smooth, blemish-free surface we all know and love.