I will be running MS Excel, preview, firefox, and activity monitor. All of a sudden my Activity monitor says that there is about % User of 50% and the fan will start running. I cannot find what program is making it run this high. I quit every program and it gets stuck on this until I restart the computer. I have done this about 3 times and it always comes back. None of the programs are registering high CPU usage. Any ideas what might be causing this or how to fix it? My last computer had an overheating problem where the fan was always on and that was part of the reason I switched to Macs - really hoping that issues doesn't follow me from Windows to Mac.