Comment 3 for bug 954831

Unfortunately no, it seems to be completely random. I don't notice that it's gone until I need to use it.

Yesterday it occurred after I experienced this bug: https://bugs.launchpad.net/ubuntu/+source/xorg/+bug/921540.

I will try and see if it happens again after that bug messes up my monitor setup. Nothing seems to help to get the systray back up and running other than logging out and in again. Just running "unity" to restart unity messes my screen up so bad I cannot use the desktop at all the apps are scattered all over the place with no window decorations.