Does it help if you use a "common" resolution such as 1280x1024 or
1024x768 instead of 1368x1024?
Well yes, but now when switching to another virtual console and
back to X, I occasionally get crashes that I did not get with the
older X server and mga driver. The screen fills with thin
colourful vertical stripes. I press SysRq+S (sync) and SysRq+U
(remount readonly), and the disk makes a sound. But then I press
SysRq+B and the computer does not reboot.
I could connect a serial console if you think that would help.