I've been seeing this problem several times a day. I can't reproduce it in any reliable way and I have no idea how I can go about collecting the needed info... Anyway, here is what happens.
I've been having this problem consistently (as in several times a day) since the move to F22. You can check out the bug report and the traces here. Currently it's assigned as an ati-driver issue, but like you seems curious that it also happens with other video (nvidia) which makes me think it's related somehow to whatever was introduced by F22 by kde... I've tried GNOME and LXQt and no problems with either of them - but I'm not the expert in this so I'm deferring to greater minds than me. ;-)
It is extremely frustrating... continues to linger. I will provide whatever type of doc required - but since I'm not the expert someone needs to advise what they need. For me, as far as I can tell it appears to be triggered by notification messages - but that could be purely anecdotal.
In the meantime I've memorized killall plasmashell ; plasmashell