7.5.5 is ok. Think it has more to do with extensions and control panels that may not be exactly compatible. My IIfx is pretty stable with 7.5.5 on it. It's also the last version that will let you drop to 24-bit addressing if necessary. 7.6 sounds nice, but I've yet to have a computer with it installed. 8.1 worked pretty well, though, but, isn't very compatible with early games, hence the need for 7.5.5...

-J