I think they all suffered from this. It's caused by a fire retardant which degrades over time when exposed to sunlight.
You might want to look into mixing up a batch of retrobrite (
http://retr0bright.wikispaces.com/).
I tried this on some old hardware I'm restoring and even with a poor mixture of the stuff, it worked very well.
You can check out my very amateur experiment with it here:
http://mjpento.dyndns.org/blogs/z80/?p=115