CalendarAgent using 100%

I'm using iCal on Yosemite connecting up to a couple of different Google Calendar accounts. I've been noticing poor system performance for a while now and was looking in the Console and see the following message a lot.

11/3/14 7:48:01.743 AM CalendarAgent[348]: [com.apple.calendar.store.log.caldav.coredav] [Refusing to parse response to PROPPATCH because of content-type: [text/html; charset=UTF-8].]

Is there a configuration setting I might have wrong somewhere? Not sure if this is causing the high CPU or not but it looks like an issue anyway.