After upgrading to Red Hat 6.1, the page-up key stopped working in the libc5 based version of Nedit. After downgrading XFree 3.3.5 to 3.3.1, the problem was resolved. (I'm using the libc5 version of Nedit from the ftp.fnal.gov because the version of glibc/lesstif version of Nedit, as supplied on PowerTools 6.1, is unstable).
I just retrieved this libc5 version, and tried it with the XFree86 3.3.5 server from 6.1, and I have no problems. Perhaps you have remapped your keyboard in some fashion, or are not using the standard Xkb extensions.