4.0 App Pool gets overloaded with same code as 2.0 App Pool

4.0 App Pool gets overloaded with same code as 2.0 App Pool

I have a very small application that is used for a high volume of short
duration sessions. During these short sessions, I call an my database, an
external web service, an external LDAP service, and then save the results
to the my database. The steps need to happen in that order and are not
asynchonous, because results from one are used to seed queries for the
next lookup. Using the same version of IIS (7), I recently deployed code
upgrades converting from a 2.0 application to a 4.0 application. With a
high volume of users in the 4.0 application, the App Pool slowed and
become unresponsive, eventually shutting down completely and pushing
traffic to the other node in the cluster.
Aside from targeting updated copies of the Microsoft Enterprise Libraries,
there were no other changes between 2.0 and 4.0 (no new connections, no
different process management, etc.). During non-peak times, both the 2.0
and 4.0 versions of the application worked, but during peak usage, the 4.0
version of the application failed so badly that the server needed
restarted (an iisreset didn't fix the problem with the App Pool).
How can I track down the problems caused by the 4.0 application? Does
anyone have any ideas about what problems that the upgrades to the 4.0
framework may cause with my application?