I've heard of this, unfortunately I don't know of 
any known methods short of sending the thing off 
to an animation restorer. Anyway - I've heard 
varying reports to air out framed cels from every 
SIX WEEKS to every TEN YEARS. Not kidding. -_-' 
Still doing research on that, but you'd probably 
want to air out the cel too often rather than not
at all...