Karl Arao has written a nice blog entry where he used also some of my tools for visualizing and understanding performance of a RAC cluster suddenly gone slow. He presents a systematic approach he used for gathering evidence and also hopefully you notice that having the ability to easily visualize performance data (with PerfSheet for example) can be very helpful and time saving when troubleshooting non-trivial problems.
Check his blog entry out here:
http://karlarao.wordpress.com/2009/06/07/diagnosing-and-resolving-gc-block-lost/
