Dell Ati Mobility does not back-store

Hi !

I have a Dell Inspiron 4000 with ATI RAge 128 Mobility (XX - some 
version) card. I have been upgrading XFree86 since 4.00 till the newest 
4.2.X and I always have the same problem.
I have a specific software that does not redraw windows upon bein 
obscured so their contents gets erased forever. Prior to XFree 4.0 the 
problem was fixed by itself, but after ver>= 4.0 one needs to specify 
the option +bs to the server. This generally works but does not work 
with the video card in my laptop.  I have tried both  +bs and -wm 
options. +bs works better but after a short while backing store does not 
wotrk at all.

Why is that and is there a fix ?

Thanks !
Mario