You didn't specify whether you are dealing with virtual or physical server....
SIM/SQL will eat up a lot of processor, mxdomainmgr will increase cpu for longer periods of time untilmately crashing the service if you are using Sql Express and have a large number (300 is not a large number) of discovered systems and/or undeleted events. Full SQL has the same problem but typically in a much higher scale. It isn't uncommon for mxdomainmgr to chew up CPU and Memory especially if SQL is local, but it shouldn't be hanging your CMS.
