kernel_task consumes 500% CPU on AC power, sometimes Typically, my kernel_task process uses 1-4% CPU and 100-700MB of memory. However, sometimes when I have the power adapter plugged in, the CPU usage jumps to around 500% (8 cores) and stays there until I unplug the power adapter. I can plug it back in and everything is fine for a few seconds or a few days, then it eventually happens again. How can I stop this from happening?
In my experience this has always been due to a hardware fault. However before doing anything else reset the SMC as detailed here: http://support.apple.com/kb/HT3964 as its power related. 

This appears to happen when the Mac is struggling to communicate with hardware. Which Mac is it? How old is the battery? Is your battery life still good? Has the Mac had any damage - dropped? Liquid? Have you tried a different power adapter?