annoying unexpected red strips on screen

[Justin Yang]

Hi, all:
I'm now suffering some "unexpected red strips" on the screen, which  
frustrates me a lot. My box runs on a sandy bridge chip, and I've  
installed the latest xorg and Intel graphics drivers related ports from  
xorg-dev. It runs quite well except the red strips shown on my screen  
unexpectedly sometimes (not always, though). I also tried the xorg from  
our official ports tree but nothing improved (even worse).

BTW, I only install the xorg related and Intel graphics drivers from  
source code (ports tree), while others, such as firefox, eclipse, are  
installed from pre-compiled packages directly from this site:
http://dev.bsdrp.net/pkg/freebsd:9:x86:32/desktop/

What I've installed:
justin at darkgeek.pts/0 ~ % pkg info | grep -i xorg
linux-f10-xorg-libs-7.4_1      Xorg libraries (Linux Fedora 10)
xorg-fonts-truetype-7.5.1      X.Org TrueType fonts
xorg-minimal-7.5.2             X.Org minimal distribution metaport
xorg-server-1.12.4,1           X.Org X server and related programs
justin at darkgeek.pts/0 ~ % pkg info | grep -i libgl
libGL-8.0.5_3                  OpenGL library that renders using GLX or DRI
libGLU-8.0.5                   OpenGL utility library
libglade2-2.6.4_5              GNOME glade library
justin at darkgeek.pts/0 ~ % pkg info | grep -i intel
xf86-video-intel-2.21.4        Driver for Intel integrated graphics  
chipsets
justin at darkgeek.pts/0 ~ % pkg info | grep -i dri
dri-8.0.5_3,2                  OpenGL hardware acceleration drivers for  
the DRI
dri2proto-2.8                  DRI2 prototype headers
libGL-8.0.5_3                  OpenGL library that renders using GLX or DRI
xf86-input-keyboard-1.6.2      X.Org keyboard input driver
xf86-input-mouse-1.8.1         X.Org mouse input driver
xf86-video-intel-2.21.4        Driver for Intel integrated graphics  
chipsets
xf86-video-vesa-2.3.2          X.Org vesa display driver
justin at darkgeek.pts/0 ~ % pkg info | grep -i libdrm
libdrm-2.4.42_1                Userspace interface to kernel Direct  
Rendering Module services
justin at darkgeek.pts/0 ~ % uname -a
FreeBSD darkgeek 9.1-RELEASE FreeBSD 9.1-RELEASE #0 r243826: Tue Dec  4  
06:55:39 UTC 2012      
root at obrian.cse.buffalo.edu:/usr/obj/usr/src/sys/GENERIC  i386

The annoying strips:
http://i50.tinypic.com/260sgac.png

And one more annoying thing:
http://i50.tinypic.com/260sgac.png

More details will be provided when needed. Any help is appreciated. Thank  
you in advance:)
-- 
Regards,
Justin Yang