That does sound pretty odd. Only real suggestion I can think of is are there any BIOS updates available for your Tyan board? Otherwise, you might have better luck finding an answer on microsoft's forums for a specific Server 2008 issue.
Tyan's own web site (http://www.tyan.com/tech/OS_Support_AMDWins.aspx) states that board hasn't been tested with Server 2008. Best bet would be to wait until testing is done and they put out a bios update.