since we're talking about network, we need network facts - not applicaton error messages.
So, could you please capture a sniffer trace and see what's going on.
Is there a RESET coming from the CSS ?
Also, I would STRONGLY recommend to upgrade to a version less than 2 years old. 5.0(45) is very very old. Go to a 6.10 and re-test making sure to capture a sniffer trace.
Be aware that the CSS does not like idle connections.
So if it detects a connection idle more than 8 sec it may kill it assuming the client is dead.
Maybe in your test with more than 50 users the idle time increases.
Regards,
Gilles.