The earlier diaply is Windows forcing standard 640x480 at 60Hz out the back. When it loads, you switch to the settings currently defined by the Display Properties function.
You could boot in Safe Mode (which is straight VGA) and change the Diaplay Properties to show only modes that the monitor supports and then pick one from that list.
Hopefully I'm on the right track for you.
Incidentally the Display Properties allow you define one monitor (laptop) as Primary and the other as Extended Desktop. Then you don't need this Fn+F5 business.
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Posting Virtuoso
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since Aug 2007