tprof is your friend

I've collected data on various 100% CPU problems and always am amazed how useful tprof data is yet it is very unintrusive.

IBM - MustGather: 100% CPU usage on AIX

Collecting data for 100% CPU usage problems with IBM® WebSphere® Application Server on the AIX® operating system. Gathering this information before calling IBM support will help familiarize you with the troubleshooting process and save you time.