The first step in pinning down a problem with any software program is to make sure you have the latest version installed. If you do not, please go to the PostNuke website, download the latest version, and follow the program's instructions on how to upgrade. Also check for security patches and releases. Many high load situations are due to hackers using a vulnerable script to execute high-load commands on the server, or to send out thousands of spam email messages. This taxes the shared servers.
One PostNuke user has suggested turning off sessions for un-registered users. This can save load time on PostNuke sites that have become very popular. Other suggestions include turning on caching in your PostNuke theme and turning on Compression in your settings. You can also try deleting modules and blocks that are not being used. Delete resource-intensive blocks, such as online block. Turn on pnRender caching and Xanthia caching (if it is used). Make sure templates re-compile and force compile are set to off in pnRender. Also try turning off pnFlashGames if you are running it. Too many concurrent users can drive up server load.
If you have done all of the above and you're still seeing a high degree of load, check your traffic statistics. It could be that you have a website that has outgrown shared hosting and you may need to upgrade to dedicated hosting.