Hi Steve,
Are you running individual app pool for every website?
Add a new site under IIS with it's own app pool. Add .html pages only and test the behaviour. If everything works fine, then create connector for this new website and add the behaviour with both .html and .cfm.
If you encounter the same issue, make sure you collect the logs. If possible, collect the verbose logs : http://helpx.adobe.com/coldfusion/kb/enable-verbose-logging-coldfusion -10.html
NOTE : Test the above strictly during maintenance window so that production will not be affected at all.
Thanks
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