The problem is your NAS is NOT following the RFC.
> 5.5.  Acct-Session-Id
> 
>    Description
> 
>       This attribute is a unique Accounting ID to make it easy to match
>       start and stop records in a log file.  The start and stop records
>       for a given session MUST have the same Acct-Session-Id.  It is
>       strongly recommended that the Acct-Session-Id be a printable ASCII
>       string.
If your NAS re-uses the Acct-Session-ID after a reboot, its not
"unique".
Very few that I know of do this (several have a work around for it,
too).
--Dale E. Reed Jr. Emerald and RadiusNT/X__________________________________________IEA Software, Inc. www.iea-software.com
For more information about this list (including removal) go to:http://www.iea-software.com/support/maillists/liststart