422 Error when using JMeter for load testing

I have a Rails 3.1 application that I am trying to use JMeter to load
test. Our site is targeted around collecting information so users
sign-up and then walk through a question and answer workflow and after
answering a few questions, they are rewarded with some useful
information. My JMeter test is configured to walk through sign-up and
then the full Q&A workflow.

Environment details:

RedHat Enterprise 6
Rails 3.1
Ruby 1.9.2p290
DB: Postgres 9

I have run tests with 20 and 100 concurrent threads and a ramp-up time
of 1 thread per second and the results are ok. However, as soon as I
decrease the rampup time so there are multiple threads spawned in a
second (even as little as 2 per second), about half of the signup calls
fail with an error 422. Once that happens, for those same threads, the
Q&A workflow breaks because we are assuming that there is a valid user
answering questions.

I'm trying to figure out if there is a problem with the application
code, server configuration, or JMeter setup.

Any thoughts or suggestions on how to troubleshoot this further would be
greatly appreciated.

Thanks,
Amit