I have Mavericks on both my imac and my macbook air. When I run Activity monitor on either computer, "Photo Stream" often takes 100% of CPU. It runs for about 5 seconds, disappears for 5 seconds, and reappears for 5 seconds. I have no use for photostream, and this doesn't seem like normal behavior. I checked that photostream was deactivated in both iphoto and system preferences. I tried disconnecting from iCloud, but did not change a thing. Any suggestions?