The problem wasn't lxdm itself it is the pclos start process.
The dm script (start of the display manager) in /rc.d/init.d is called in every runlevel exactly the problem is the call on runlevel 3.
we need to delete the call there. after this it is working well but there isnt a big difference between the ram usage in lxdm and gdm.
Now one question: Does we get trouble with other things when deleting the dm call on runlevel 3? tryed gdm after the change it works also well like before.
The init script dm (/etc/init.d/dm), in turn will start another script: /etc/X11/prefdm, the latter already include support for various dm: gdm, gnome, kdm, kde, wdm, xdm, slim, lxdm
The condition to start lxdm, is that it should be defined in /etc/sysconfig/desktop as DISPLAYMANAGER=lxdm, alternatively, if DISPLAYMANAGER is not defined, it look at DESKTOP, allowing for the same values.
Look at the scripts for further info.
The fact that dm is active at runlevel 3 is not correct, as it should run only at level 5 ... but the issue probably is only a side effect of a wrong invocation of lxdm