How many concurrent connections that an ACS server version 4.2 latest patch can handle?

I have about 50 routers and layer-3 switches that autheticate via tacacs+.  The AAA server used to be on a Linux machine running open-source tacacs+ built by me.  I have a perl script that will log into all 50 devices at the same time to collect statistics.  This script is multi-threaded.  Everything is working fine so far.
I recently out-sourced the AAA function to a 3rd party company, not by my choice.  The 3rd party uses Cisco ACS version 4.2 with the latest patch running on Windows 2003 Enterprise Server with 16GB RAM and quad processors with quad-cores, IBM x3650-M2 hardware. The connectivity between the 3rd party and my company is through a DS-3 connection.  Maximum bandwidth over this DS-3 connection is less than 10Mbps at most.
I noticed that for the past 3 months I have multiple failures with this perl script due to authentication failure with the ACS server.  If I just run the script again a few routers/switches, there are no issues; however, whenever I started the script to log into 50 devices all at the same time, it will fail.  If I made the configuration on all routers/switches to point back to the old open-source tacacs+ server, the issue goes away.  The minute I switched back to the
new ACS server, the issue came back.  If I modified the script to hit one device at a time, it works fine.  I think it is the ACS server can not handle a lot
of AAA requests at the same time.
Does anyone know how many concurrent connections that an ACS 4.2, with latest patches on Windows 2003 Enterprise Server with lot of memory and CPU power, can handle?  I can't seem to find this anywhere on Cisco website.
Thanks in advance.

No, Im not saying ACS cannot cope.
Concurrency and latency are very different things. ACS CSTacacs can handle many 100s of simple authentications/authorisations per second with users in the internal database. If 1000s of devices all send traffic in the same instant it would take some seconds to work through the backlog of traffic.
Also, worth considering that a limited number of tasks within ACS (or threads) can actually handle a much greater number of "logins" because they are generally multi-message allowing ACS to keep lots of plates spinning.
If users are in an external databases the latency (per authentication) can increase depending on where the users are (eg Windows AD) and if bad enough can have a serious effect on the overall authentication rate. At which point customers normally turn to load balancing.
If your device timeouts are 20 seconds (totally reasonable) I suggest the issue is more likely to be something else... a bug, perhaps specific to v4.2?

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