if you're not using a relatively multi-core demanding application, then things like word processing, web browsing, music and video isn't going to use them all up.
What are you doing when that happens? It is possible that you are running a program or app that only uses 1 core.
Usually when I am playing a game I can monitor the cpu activity and it usually looks good for about 10 minutes. Then the first cpu just goes to like 100% and all the others are like at 40%. I think it might be overheating because it is pretty hot in my basement but I am not really sure.