The ps command shows many repeating processes named "app_process". Why?
I had a similar problem before. In my case, the culprit was one or more background services. Deleting Google Goggles solved it for a while, then it came back.

This time, I removed Trust Go. So far, so good.

Whether or not you have those specific apps, I'd say there's a fair possibility that one of them is the problem.

I tracked it down by going to Settings > Developer Options and checking the box to overlay the processes on the screen. This is important because the act of using another app to check on what's happening can itself change things. I saw that even though a system process was eating all the CPU, Trust Go was frequently in the top five. Removing it seemed to cure it for me