The Z drivers often show behaviour like that. In Vista or XP when I create a custom size and save that custom size it will fall back to another size or the size set in preferences when the driver is closed. I go back and reselect the custom size name again and then it will stick. I use Qimage to print from so it may be worse with CS5.
A way to get driver settings more stable is creating printing shortcuts in the driver and selecting them instead of temporary driver setting changes.
For a default setting like "let application manage color" that should be permanent, change the default setting with printer preferences.
met vriendelijke groeten, Ernst Dinkla