Does your Activity Monitor CPU usage add up to 100%?
I have returned three 27" i7 iMacs because Activity Monitor's %Idle: keeps changing from 48% to 98%, while the %User: and %System: often add up to less than 3%.
Most of the time, the total usage shown adds up to less than 55 %.
Where is the other 45% of the CPU that I paid for? It might be running in the background. These computers have been running way hotter than the iMacs I checked in the Apple Store.
Apple Care doesn't have a clue. They just declare the computer DOA and tell me to ship it back.