I solved this problem on two Ironlake systems by using uxa acceleration instead of sna.
The problem seems to exist with certain combinations of kernels and xserver-xorg-intel for computers before Sandy Bridge.
Create /etc/X11/xorg.conf.d/20-intel.conf with contents:
Section "Device"
Identifier "card0"
Driver "intel"
Option "AccelMethod" "UXA"
EndSection