CentOS 5 :: X Windows Applications "Can't Open Display" After Su

I have recently resurrected an old X Windows application and I am having a problem getting a CentOS 5.3 client to run the application from a CentOS 5.3 host.

If I am logged in as root to the client side and ssh -Y over to the host, then run xclock, everything works just fine.

However, if after I ssh -Y over to the host, I then su - userx on the host, and then run xclock, I always get the "Can't open display" error.

I have tried:

Running xhost + on the client. doing the "xauth add display MIT-COOKIE xxxxxxxxxxxx" on the client & host, both as the root user, and as the userx. I can't seem to get a definitive answer as to which user and which machine (client/host) this needs to be run as.

Also, when you "xauth list $DISPLAY" you get nothing, (because it is either localhost:0.0 as root, or :0.0 as the su - user) you have to "xauth list", and then you get a list of cookies, but since the "echo $DISPLAY" doesn't match any of them, it is not obvious which one to use. Also, they are ALL "host/unix:X.0", so I assume they are all socket based, and as such I would assume that the their cookies would not be usable for remote TCP connections?

I also saw information about using netstat -anp to look for 60** LISTENERS, I could not find any on the client or host sides.

Nothing seems to work.

I heard about a program sux that is on ubuntu, but I could not find it for CentOS. Is there an equivalent on CentOS?

I have searched the web and found lots of info in forums all over the net with what appears to be the correct way to do this, still nothing appears to work.

What is confusing is that if the TCP LISTENERS were not there, I should not be able to get it to work as root either, so it must be in the transition of the su - userx.