issue in unifying domain.com with www.domain.com using 301 redirect

Hi,

My company website is with ASP.NET 2.0 and one day the marketing guy ask me to unify domain.com to www.domain.com so that if visitors visit domain.com they will be redirected to www.domain.com and the status code must be 301 (301 Moved Permanently). He asks me to verify with http://www.redirect-checker.org/. In this site if we input domain.com and click Analyse button, if we have "Status: 301 Moved Permanently" then it's ok.

I tried with so many methods (using URL_Rewrite muldule with Canonical redirect, setup domain.com and redirect to www.domain.com, add rule to web.config file) and all of the methods work fine. However, I only have "Status: 301 Moved Permanently" for the first times access to the site. A second times when I access I see the site be redirected correctly to www.domain.com but the header is "HTTP/1.1 200 OK" but NOT "Status: 301 Moved Permanently" anymore.

Could you please help me how to always have "Status: 301 Moved Permanently"?

Thank you in advance