Fixing flaky paint

We have a 3 year old house where the builder's decorators have done a poor job on the first painting of the plasterboard walls. Now that I'm getting round to redecorating, several areas of paintwork show really bad adhesion and start to flake off when they are painted over. Is there any way to remedy this without resorting to removing the bad paint - I was thinking about applying a very dilute PVA wash over the bad bits, and hoping that this would penetrate the paint layer. (The paint does seem to be a bit soluble already, as wiping with a damp cloth does take some off.)
Ideas?
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