GetModeLine does not occur in my /var/log/XFree86.0.log file, even after severel hours of operation. I do, however, occasionally see the "flicker" sometimes it shows up just after a switch virtual desktops. Other times not at all.
At the office I run with two displays: the laptops LCD, and an external LCD connected to the back. So far as I've noticed, the external display never exhibits the flicker.