I had a problem with the availability index being too low on a fast server. take a look at this article:
http://blog.nashcom.de/nashcomblog.nsf/dx/configuring-sai-loadmon-for-fast-servers.htm?opendocument&comments
and these notes.ini settings:
Server_MinPossibleTransTime=1500
Server_MaxPossibleTransTime=20000000
after you add the settings, shutdown the server, delete loadmon.ncf file, then restart the server
then use the 'show ai' command when the server is under load to get a recommendation for the Server_Transinfo_Range.
then see if the index behaves itself better....