All,
I am looking for explanation of very strange behavior we are experiencing with SQL server.

Preamble:
We have web application which requires log in. Then it runs SELECT statement checks users table and if success it UPDATE logged in field with 1 or 0 if you log out.
We also have different but very similar portals pointing to different databases within one SQL server. All of them are running the same login code and working fine.

Mystery:
It happens only for one database for one table and according to SQL profiler only with one command which is UPDATE updating logged in field I described above, it hangs database.
If you run the last UPDATE in Query Analyzer it takes 40-47 seconds to run and seems like unlocks everything back and you can have fun with this until the next time. It may happen within 5 minutes or an hour or two again.
Last night we restarted SQL Server, then closed shop and went home. None of us were using it till 8 am this morning.
When we try to log in it stock again. Only for this database, all others are working fine.

Please help:
We need to solve it ASP. We will be grateful for any suggestion, any ideas.

Thanks in advance
Valentin