There appears to be 3 BIOSs here you could try:
ftp://ftp.chaintech.com.tw/bios/5agm2/
dated
5agm2309.exe - 03/09/99
5agm2614.exe - 6/14/99
5agm2ncra.zip - 08/24/98
There's also 2 more from the Chaintech website:
11/02/99
http://www.chaintech.com.tw/FILES/5xMB/5AGM2B02.EXE
8/11/99
http://www.chaintech.com.tw/FILES/5xMB/5AGM2811.EXE