If you have an HP computer, (maybe other makes as well), you will probably have a process running from start-up called "kbd.exe".
I was experiencing the same as you some time last year, and the cause was kbd.exe reacting with one of the svchost.exe's and maxing out the CPU.
kbd.exe:QUOTE
kbd.exe is a process from Logitech which allows you to configure the additional keys on their multimedia keyboard products. This is a non-essential process. Disabling or enabling it is down to user preference.
Try disabling it, to see if it's the culprit, or, leave things as they are, and when it happens again, open task manager, Ctrl+Alt+Delete, and stop it running. You soon find out if it is the cause.