how to force libxcb version below 1.4 when re-installing X (was: [gentoo-user] my xorg-server 1.6 seems a bit unstable - what am I doing wrong?)

[Denis]

I cleared everything X related from my box, and it is still alive and well :-)

Now, as I was explaining in my previous thread, I would like to
re-install X and all related apps, but forcing the older libxcb
version.  After I upgraded to libxcb 1.4 a few days ago, my X was
unstable and kept crashing while scrolling in Mathematica, so I wanted
to clear everything X out, start from scratch, and revert to my
original X configuration.

Here is the new problem.

In my package.mask, I specify:

>=x11-libs/libxcb-1.4
>=x11-base/xorg-server-1.6
>=x11-base/xorg-x11-7.4

My USE is now:  "alsa ao bash-completion bzip2 cdda cddb cdparanoia
cdr cpudetection ieee1394 lame ldap libwww
                           mad mmx mmxext mp3 openal oss ppds rtc smp
spell sse sse2 tta usb wavpack winbind xml -dri -isdnlog -pppd"

So, I put in the X USE flag and attempt

emerge -NDavu xorg-x11

and get

!!! All ebuilds that could satisfy ">=x11-libs/libxcb-1.2" have been masked.
!!! One of the following masked packages is required to complete your request:
 - x11-libs/libxcb-1.4-r1 (masked by: package.mask)

(dependency required by "x11-libs/libX11-1.2.2" [ebuild])
(dependency required by "x11-libs/libXext-1.0.5" [ebuild])
(dependency required by "x11-libs/libXv-1.0.4" [ebuild])
(dependency required by "x11-base/xorg-x11-7.2" [ebuild])
(dependency required by "xorg-x11" [argument])

What gives?  Am I doing something stupid (well, aside from trying to
downgrade...), or is there a way for me to fix this by some keywording
trickery or something to that effect?  Help, please :-)