Can't start X apps from su

[Andrej Prsa <andrej.prsa@fmf.uni-lj.si>]

Hello List,

I have a very-difficult-to-reproduce-but-pertaining problem with
accessing X applications (such as nedit) when gaining access via
su from some other user's X session. If I log in as root to X,
such problem never occurs. But as I don't want to take the name
of root in vain, I got used to using su. *Usually* things work,
but suddenly, after an indefinite amount of time, it just doesn't
work any more with the only statement that it cannot connect to
X session. As a side-effect calling mc from su session becomes
miraculously black-and-white. If I close all open terminals,
restart one and go into another session of su, I could sometimes
regain normal su operation and sometimes not. This has been
bugging me since I installed Debian (oh, it's 3.0r1 stable), but
I lived with it until now...

Thanks for your time,

Andrej