webservice error in NAV 3 Tier Architecture

Dear All,

I installed the SQL server in windows server 2008 and installed NAV server in windows server 2003.I create a web service in NAV Server and call the web service which is available in asp.net. My Asp.net code:
………
ApprovedPurchasePrice_Service tt = new ApprovedPurchasePrice_Service();
tt.UseDefaultCredentials = false;
tt.Url = "http://xx.xx.xx.xx:7047/DynamicsNAV/WS/XXX/Page/ApprovedPurchasePrice";
tt.Credentials = new System.Net.NetworkCredential("xx", "xx", "xx");

ApprovedPurchasePrice_Filter[] _filter = new ApprovedPurchasePrice_Filter[1];

ApprovedPurchasePrice_Filter nameFilter = new ApprovedPurchasePrice_Filter();
nameFilter.Field = ApprovedPurchasePrice_Fields.Ending_Date;
nameFilter.Criteria = "06-2010-23";
……

Because of the error format of the Date Time , the error “06-2010-23 is invalid” occurs. After I correct the date time format, I debug the web and find the web service is not available.The error message is “the logon fail to connect to the server xx.xx.xx.xx”.The HTTP 500 error occurs when I open the "http://xx.xx.xx.xx:7047/DynamicsNAV/WS/XXX/Page/ApprovedPurchasePrice" in IE.

It's very strange!

I try to open the "http://localhost:7047/DynamicsNAV/WS/XXX/Page/ApprovedPurchasePrice" in IE and find the "http://localhost:7047 /..." web service is available.

After that operation, I open "http://xx.xx.xx.xx:7047/DynamicsNAV/WS/XXX/Page/ApprovedPurchasePrice" in IE again and find the web service is available again!

Can you tell me how to fix it?

Thanks!