It happened to me with one Clip Zip (8Gb grey). It was fine for a few weeks, then it began to lose capacity when powered off (like 20%/day).
I tested that with old rockboxe versions, and the original firmware : it was the same. I have no idea about the cause, and I didnt find any solution.
Since then, I bought two other Clip Zip (4Gb black, 4Gb blue), and no problem with them after a few months.
Before these clip zips, I had two clip+ with buffering noises ...
A bit sad about their build quality, but there's no real alternative.