The system process (not the system idle) can go to high CPU use. To narrow down what is causing this you need to narrow it down to a thread that system is managing.
Do this by downloading the System process explorer from Microsoft. Download and run the procexp.exe program that the .zip file contains. click “run” and then “run” again on the dialogs (you basically run this program – not install it).
It runs and display a more complex system overview than the taskmanager. Sort on the “Process” column and then find and right-click the “System” process and pick “Properties”.
You can now pick the “Thread” tab and sort on the CPU column and find out which driver is running the high CPU. To see where and what version this file is click the line e.g. amdk8.sys and then click the “module” button. It will pop up a normal File Properties dialog box.
You have all you need now to search technical help sites or your hardware provider to see if there is updated software for this driver: searching for “amdk8.sys high cpu” is a lot better than searching for just “system high cpu”