This one might save someone some searching :-)
I can't remember when it started happening but the UserEventAgent process started using 90 to 90% CPU so basically stealing one core all the time.
I thought it was related to a whole lot of things that turned out to be red herrings.
In my case it was TeamViewer causing it. Uninstall TeamViewer and it spikes but generally sits at 0% rather than 95%.
Some Googling found one particular plug in related to certificate verification that the UserEventAgent uses caused their high CPU usage so maybe some Apps just make it go nuts.