Can you FULLY list what is displayed in system tray (if you have auto-hide turned on, click the little arrow pointing to the left) , and also , click Start, Run , enter "MSCONFIG", click OK, on to the startup tab, what is ticked there?
If none if these turn out to be culprits, might be worth going to task manager, clicking the processes tab, and sorting the processes by the CPU column, so the process suing most processor time is at the top of the list. Next time it mis-behaves, watch the list, and see if any process suddenly spikes its usage...