The server is unavailable. at System.DirectoryServices.Protocols.LdapConnection.BindHelper

Hi All, I am getting the following error once in a while (not always).I researched the server log no luck,I tried producing it by myself no luck.I would pleased if some has any clue.

This is heappning while fetching 
Membership.ValidateUser(username, password)

The server is unavailable.    at System.DirectoryServices.Protocols.LdapConnection.BindHelper(NetworkCredential newCredential, Boolean needSetCredential)
   at System.Web.Security.ActiveDirectoryMembershipProvider.ValidateCredentials(String username, String password)
   at System.Web.Security.ActiveDirectoryMembershipProvider.ValidateUserCore(String username, String password)
   at System.Web.Security.ActiveDirectoryMembershipProvider.ValidateUser(String username, String password)
   at Ika.TechFwrk.ADAM.IkaMembershipProvider.ValidateUser(String username, String password)
   ...