download process explorer and see if you can gather more information and see which applications/open files are causing the usage to spike...

http://technet.microsoft.com/en-us/s.../bb896653.aspx

[edit] and of course ensure all updates/patches are applied as JPnyc has pointed out in his post, there was a WUA issue which displayed the exact same problems you are describing, but this was fixed in a MS patch [/edit]