DLL issue - Application picks up the older DLL

Dear Team,

I have Window 2008 R2 server with .net 3.5 F/W. My WCF application is hosted in IIS 7.0. I always release release related DLL (not all)  to production - and restart the IIS web site.  All DLLs are local ( No GAC)

UI calls the WCF services and get the data. Sometimes, instead of picking up new DLL, WCF services picks up OLD dlls.  

Note : I always  delete all temp files and restart services.

Please suggest avoiding DLL issue.

Regards,

Mitesh Sheth