Both QA and app use cache, but the way the cache gets used is that exactly the same command needs to run, down to the whitespace. This means that it's possible when you run from QA, you might force a recompile or a different plan.

Frank,

We actually saw this at Andy's old company during TechEd 05. We queried Christian Kleinerman of the storage engine and a few other MS guys at the time. They said that at times they've seen a plan mistakenly flushed and then a bad one chosen under load, especially memory pressure. They didn't have a good way to fix it other than flush or restart the server. That seemed to work for Andy and so they did that.

If you do that, one suggestion they had was to set a startup script that runs some important queries before the app to try and build a cache up. As mentioned, be sure that you use the same query that's expected.

More reading: http://blogs.msdn.com/sqlprogrammability/archive/2007/01/23/4-0-useful-queries-on-dmv-s-to-understand-plan-cache-behavior.aspx