Our enterprise is having some problem when the number of incoming request goes beyond a certain amount.

To make things simpler, we have N websites that uses, amongst other, a local web service. This service is hosted by IIS, and it's a .NET 4.0 (C#) application executed in a farm. It's REST-oriented, built around OpenRasta.

As already mentioned, by stress testing it with JMeter, we've found that beyond a certain amount of request the service's performance drop.

Anyway, this service is, amongst other, a client itself of other 3 distinct web services and also a client for a DB server, so it's not very clear what really is the culprit of this abrupt decay. In turn, these 3 other web services are installed in our farm too, and client of other DB servers (and services, possibly, that are out of my team control).

What strategy do you suggest to try to locate where the bottleneck(s) are? Do you have any high-level suggestions?