It might be bad parameter sniffing. You can read about it on many articles.
For more information specific to your problem, check the following article: http://www.sqlservercentral.com/articles/SQLServerCentral/66909/
Luis C.
General Disclaimer:
Are you seriously taking the advice and code from someone from the internet without testing it? Do you at least understand it? Or can it easily kill your server?
How to post data/code on a forum to get the best help: Option 1 / Option 2