screen ping-pong

[Jean-Marc Ranger <jmranger@hotmail.com>]

Hello,

Looking for some pointers for a longstanding intermittent issue (first 
seen early this summer).

Basic setup info:
- Dell Lattitude E6400 (Intel x4500 video) connected to an external 
display, using DisplayPort at both ends
- Up-to-date Wheezy, with some packages from Backports, including kernel 
3.10 - although the problem also happened with 3.2


Issue:
- Upon screen awakening (the computer itself isn't sleeping), the screen 
won't wakeup in X mode. Switching to console (ctrl-alt-F1), both screens 
are on.
- It is possible to wakeup the screen using the laptop' "switch display" 
hotkey (Fn-F8), although in those cases, the image may start to bounce 
from the internal display to the external one, to both, etc.
- The bouncing usually stops after 20-60s.
- At that time, the mouse is very sluggish. Top shows sometimes (but not 
always) that Xorg is using 100% CPU on one of the cores.
- From that point, the bouncing may or may not restart anytime.

I understand that much details are missing. At this point, I'm 
especially looking for:
- links to similar issues
- tools that I could use to get a better understanding of what's going on

Thanks,

Jean-Marc