SQL Server stored procedures fast in SQL but slow when called by ASP.NET
Changing ARITHABORT inside the procedure causes it to recompile:

http://support.microsoft.com/kb/243586

So most likely you're dealing with parameter sniffing. If you don't figure other ways to fix this, adding "... option (recompile)" into the end of the statement you're having trouble should solve the issue. That of course increases CPU usage (slightly) because it has to do a new plan for that statement every time.