When you open the Task Manager > Processes tab, two things to do;

1] Click on the CPU column either just once or twice. The object is to list the processes in 'reverse order', eg most usage at the bottom. That way you will see more easily the heaviest process usage as it happens.

2] If that Process tab has an option at the bottom of the page to "Show processes from all users", click that. Not all processes are shown until you select that. Then scroll down, if you have to, to see the bottom entries and look for any heavy usage processes.

Mark