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To reproduce that you need to start tomcat running any jsp, stop that from Runtime tab and repeat these steps few time ( usually more than 3) as fast as you can. Sometimes it says that Tomcat is running, but external browser doesn't start and IDE takes 99% CPU time. Only killing IDE helps.
Can you attach a thread dump?
Tomcat4 plugin was replaced by Tomcat5 plugin. This issue isn't relevant to new plugin.