Just to be sure that I have this straight;
You are having trouble with your Netscape web based e-mail account using Netscape web browser.
I would start by clearing out your cache folder.
For 4.X, it should be under program files\netscape\users\xyz\cache (xyz = your user name)
For 6.x, it is under Windows\application data\mozilla\***\cache (*** = a code for your user profile)
Another thing to check is the settings of the Netscape Mail and Newsgroup program as it will tie itself into your netscape web mail account.
I would suggest that you consider the k-meleon web browser. www.kmeleon.sourceforge.net
It operates very similar to Netscape 4.X with the 6.X updates. It loads my Netscape mail around 40% faster than Netscape 6.1
You will need to download and install Java from Sun. As the password dialog for netscape mail is a Java applet.
Perhaps the best feature of K-Meleon is the built in pop-up window stopper.
The only negative that I have seen is that it can only import 512 IE favorites.