After upgrading to RedHat 5.2, I had no problems with netcfg. Then I updated to XFree86, version 3.3.3.1-1, as recommended. Now, when I execute netcfg for the first time after logging in, to connect to my ISP via a PPP dialup connection, all attempts to start X clients fail with an error message complaining that they do not have permission to use display ":0.0". A rather annoying workaround is to restart X without logging out and then re-connecting to my ISP via netcfg.
I found a workaround. I used the xhost command and added my local host name.
I suggest that you use xauth instead, because it is much more secure. Do a query on www.dejanews.com on "xhost" to see horror stories. Future releases of Red Hat Linux will not change the hostname for dialup PPP and SLIP connections, as we have decided that this is not safe for all programs, X being the most notable example.