Delay When SharePoint 2007 Calls a .Net web service

We have a SharePoint 2007 site which has custom user controls hosted on SharePoint pages as smart webparts. The user controls call a .net web service on another server. The site was working fine until suddenly all request to the web service are delaying about 20 seconds, and after doing some investigation it turned out that the web service is responding fine, but the requests reside in the SharePoint server for the delay period then it invokes the web service (noticed that using network monitoring tools).

We were checking everything for the past 2 days with no 开发者_高级运维success, so can anyone help please