kernel_task uses up over 50% cpu usage until I open Activity Monitor
Maybe you could get a CPU monitor to check the status beside the Activity Monitor. This way you could tell if its really tied to activity monitor. I have an app called  CPU LED which shows a string of colors indicating CPU usage but there are several in addition to this. Doing an app store search on CPU Monitor brings up about 50 hits. You can scan through these to find one that suits your needs. Then if it is tied to Activity Monitor you could download a mac virus checker with a free trial period and run it on your machine.