Problem in running NAV2009 with SQL Server 2008

Hi everyone.
Not sure if this is the right place to post my problem here. It does look more likely a SQL Server problem :?
Btw, my customer is using SQL Server 2008 and I have recently installed NAV2009 on it.
Installation was fine, and I could connect to the server DB from a remote client with SA password (database level of course).
Two more users tried connecting the same way and for 90% of the time, only 2 users could be connected to the DB with SA pwd.
Then I added 2 users in the DB server directly, granting them access to the NAV DB only.
So far so good... but about 30-45 mins later, suddenly no one could get access to the NAV 2009 DB, with whatever login. SA failed, those newly created ones failed. The error msg was "Invalid UserID and Password".
I have never experienced such problem in NAV4.0/5.0 with SQL Server 2005.

Just want to know if someone is hitting the same problem as me...
pretty frustrated indeed.. ](*,)