After adj. cost,posting for any transaction hangs NAV2009

Dear all,

I am not sure if you have faced this problem before.

Background:
NAV 2009 Classic
SQL server 2008 64 bit
Windows server 2008 64 bit
16GB RAM

Situation:
We face a situation where any transactions that require posting i.e transfer order, Purchase receipt etc, causes NAV client to hang. This happens after we have run adjust cost over the weekend.

When we looked at NAV, adjust cost program seems to have completed. But when users starts posting transactions, the posting window will appear but it will not close and it will just hang and we need to force close and start NAV again. When we start, the transaction that was supposed to be posted, was posted.

Another observation. When we run task manager, the sqlsvr.exe is taking 15.7GB RAM out of the 16GB RAM.
What we have done to try:

1. We have re installed SQL Server. It doesnt solve the problem.
2. Restarted the NAV Server. Sqlsvr.exe still high. And posting problem is not solved.
3. Back up NAV and restore. First time it worked. Subsequently no.

Sometimes, it just solves by itself without us doing anything.

Any one has any advise of the possible issue?

thank you. Appreciate your urgent advices.

Jap