ASP.NET 1.1 Error: System.Net.WebException: The request failed with HTTP status 401: Unauthorized

Hi,

I have an old XML based web service (ASMX) running under ASP.NET v1.1.4322 on Windows Server 2003 R2 SP2 Standard Edition.  This web service was running fine since last so many years under a IUSR_XXX anonymous account.  But in the past few weeks, I've noticed that all of a sudden every 2-3 days this web service throws the error "System.Net.WebException: The request failed with HTTP status 401: Unauthorized".  Then I have to restart the machine to get it working again, but it again happens in next 2-3 days. 

Any idea what could be causing this sudden issue ? This Windows server is located in a secure data center and nobody can access it without approval.  Is there any log / trace that I can check for any clue ?  

Thanks,

VG