ok, searching launchpad and googling a lot, 
I found this bug is an effect caused by bug: 
https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-intel/+bug/438000
also reported into bug on freedesktop.org: 
http://bugs.freedesktop.org/show_bug.cgi?id=22076

As you can see the real problem is a bug/limitation on mesa-lib, you can
avoid it by putting your external Monitor (VGA1) "below" your current
screen (LDVS1),

NOTE: You've always "2048x2048 maximum 3D texture size" limitation so
check if your vertical size will not overcome 2048, example: In my case
vertical size is: 640 +  1024 =  1664 that doesn't overcome 2048 :-)

I've tested it and now it works great with compiz too,

this an example script that you can use to switch correctly on (without
freeze) your external monitor:

#!/bin/bash

lvdsres="1024x600"
vgares="1280x1024"

if [ -e ~/.vgaconnected ]
then
xrandr --output VGA1 --off --output LVDS1 --mode $lvdsres && rm ~/.vgaconnected
else
xrandr --output LVDS1 --mode $lvdsres --output VGA1 --mode $vgares --rate 60 
--below LVDS1 &> ~/.xrand.log && touch ~/.vgaconnected
fi

Let me know if this solution solved your problems too!

bye.

** Bug watch added: freedesktop.org Bugzilla #22076
   https://bugs.freedesktop.org/show_bug.cgi?id=22076

-- 
karmic gnome-display-properties hangs for external monitor mode 1280x1024
https://bugs.launchpad.net/bugs/445100
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