Intermittent HTTP 404 error when invoke a ASP.NET webservice deployed on IIS7

Hi

I have an APS.NET webservice deployed on IIS7, Windows Server 2008 R2. It works fine in my dev env (7 calls per second). And in production server, it will be invoked once every 2 seconds at runtime. The strange thing is about 1/3 calls can return successfully and about 2/3 calls return error saying "The request failed with HTTP status 404: Not Found.". I don't think the url is wrong or something is missing, since 1/3 calls can return success. Is there any setting i can check, or any log i can look at?

BTW, i use Mutex in my webservice, is this a possible reason?