Server not Operational Error in Memberhip provider GetUser method

Hi am using ASP.Net membership provider and the GetUser method intermittently throws "The server is not operational" error.

On performing IISReset, the error disappears.

The server is not operational.
(System.Runtime.InteropServices.COMException)

at System.DirectoryServices.DirectoryEntry.Bind(Boolean throwIfFail)
at System.DirectoryServices.DirectoryEntry.Bind()
at System.DirectoryServices.DirectoryEntry.get_AdsObject()
at System.DirectoryServices.DirectorySearcher.FindAll(Boolean findMoreThanOne)
at System.DirectoryServices.DirectorySearcher.FindOne()
at System.Web.Security.ActiveDirectoryMembershipProvider.FindUser(DirectoryEntry containerEntry, String filter, SearchScope searchScope, Boolean retrieveSAMAccountName, DirectoryEntry& userEntry, Boolean& resetBadPasswordAnswerAttributes, String& sAMAccountName)
at System.Web.Security.ActiveDirectoryMembershipProvider.GetUser(String username, Boolean userIsOnline)
at Deloitte.Ca.CommonLibrary.ADMembershipProvider.GetUserExt(String userName, Boolean userIsOnline)
at Deloitte.Ca.CommonLibrary.ADMembershipUser.get_Current()

Connection string and membership provider I am using is this: 

ConnectionString

 <connectionStrings>

    <add name="ADConnectionString" connectionString="LDAP://domain.subdomain.com/" />

</connectionStrings>

Membership

<membership defaultProvider="i">
<providers>
<add name="i" type="Microsoft.SharePoint.Administration.Claims.SPClaimsAuthMembershipProvider, Microsoft.SharePoint, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" />
<add name="MembershipADProvider" type="CommonLibrary.ADMembershipProvider,CommonLibrary, Version=1.0.0.0, Culture=neutral, PublicKeyToken=d0ede0d092fc94f9" connectionStringName="ADConnectionString" connectionUsername="username" connectionPassword="password" applicationName="/" enableSearchMethods="True" description="Default AD connection" attributeMapUsername="sAMAccountName" />
</providers>

</membership>