KDE panel artifacts....?

Have a prob in KDE, where there are strange “artifacts” over the panel that stop me from being able to click on stuff.
eg, as per this pic: https://dl.dropboxusercontent.com/u/2512459/KDEpanelIssue.jpg there are a couple of ‘lines’ over the devices icon that make it impossible to click on the icon. Is only intermittent - seems to happen for a while every other day…??
Cannot figure out what it is - in panel config there is nothing, and it doesn’t appear to be a random icon on the screen, as clicking/dragging etc does nothing…
Anyone come across this? Probably a simple solution, but can’t figure it out…

Having no idea whatsoever of the cause, I’d try resizing the panel to see if the artifacts resolve into something that may give a clue to it’s nature. Also, whats your video card and are you using open source (intel) or proprietary video driver? Disabling/enabling 3D effects or compositing method (opengl x xrender in 13.2, not sure about 42.1) make any difference?