You don't have to worry about that because there is nothing you can do.
First of all you have to remember that NS 6 has very little in common with NS 4 -these are two completely different browsers.
NS 6 is still in its beta stage (after more then a year) and apparently AOL or other current owner doesn't have money to invest in this piece of junk.

I have the same problem with NS 6. On many pages rollovers are rendered inconsistently, i.e. sometimes they do , sometimes they don't. Sometimes instead of the image I get part of the browser toolbar (how about that?).
I use IE 5.5, both version of Netscape, Opera and Web TV to check my pages in but most of the fun I have with Netscape 6 (sorry, Web TV is the worst).
So don't be surprised with NS 6 it's probably not your code but the software itself.

Alex.