I was sick and tired of looking at a nasty looking Amiga with yellow all over it.  I said what the hell and tried somthing new.
Get a bottle of Soft Scrub with Bleach.  Get a Dish Scrub Brush.  Apply it all over the plastic and rub it like hell.  I had to run the brush under the hot water to thin the soft scrub out a bit but it got the thing looking beautiful.....dare I say it almost looked brand new.  Give it a try.