Comment 4 for bug 1509326

It's my main and only work machine, I can upgrade if and only if I can easily revert any upgrade.
I don't expect to be able to revert from Wily to Vivid, so that's not an option unless I can have guarantees.
A driver update would be fine, I can always go back to Nouveau, then reinstall the current Nvidia driver.

Given #1488206 and #1507328, and that I've been having these problems for years now, I do not expect the driver update to fix anything, but I can still check and see what's happening. (My bets are on a race condition somewhere in the wakeup software stack since the symptoms are so irreproducible, but I don't know how to validate that.)

More symptoms:

9) Sometimes (rarely), X stops recognizing the screen. I get a "default" screen @1024x768 instead of the VGA-0, LVDS-0, and DP0...DP-6 connections in xrandr --verbose. It does not help to reboot, not even when disconnecting from all power including battery. What does help is installing Nouveau, then reinstalling nvidia-current.

10) Sometimes (slightly more often than symptom nr. 9), I get the same as nr. 8, shutdown does not complete. I cannot get a console through Ctrl-Alt-F1, but I can Ctrl-Alt-F7 and get a graphic screen with a greyed-out desktop and an empty dialog window (there's a white title bar and a grey dialog area, but no title, no icons, no text, no buttons, no border, entirely unresponsive).
What I could do was to Ctrl-Alt-Del to get to the lock screen. From there, I could resume my normal session (just to restore the same greyed-out-with-unresponsible-dialog behaviour). I could also start a guest session, but I didn't try anything there because I believe guest isn't in the sudoers list.
Logging in with explicit user name gave me a black screen and unresponsiveness to Ctrl-Alt-F1 and Ctrl-Alt-F7, so I power cycled the machine and called it a day.