[Users] system busy - scenario

[unplug]

I have experienced an error with using openser 1.0.1 + mediaproxy 1.4.2.

The system works fine in the following loading.
User login to the system: 20 hardware phones
concurrent RTP session: 5

However, when I increase number of user to 50 and concurrent RTP
session to 30.  After a few minutes, the system is very unstable.  SIP
phones display "wait logon" message as they can't logon to the system
in some reasons and no one can make call even some users can logon.
This situation will remain few minutes and it will resume normal.
After a while, system will become unstable again.

Anyone has such experience before?  I have grepped the log using ngrep
but it is very difficult to troubleshoot as it is a log of whole
system with 50 users.  In my simple analyst, I found that the most
error replies happened in the log are listed below.
407 - proxy authentication required
408 - request timeout
481 - call leg transaction does not exit
Do above errors will make the system unstable?  Which one is the most
critical?  When and how the above errors will occur?

To repeat the situation above, it involves a lot of resources.  Is
there any sip phone simulator which can be used to simulate as a large
number of user to create calls simultaneously?

Thanks!!