Thank you for the excellent bug report, unfortunately I can't see anything obviously wrong with your system specs that would cause the problem.
Could you try testing other games on Instantaction please? That way we can see if its a legions-specific issue or not.
In terms of troubleshooting this appears to be an IA-side issue so the support site at
http://support.instantaction.com may be able to help, especially if you place a ticket with them.