Authentication Timeout on Invalid Password

Hi,

We have a problem with a couple of our DCs that are no longer returning the correct error message on an invalid password attempt (invalid username is fine).

Working DCs are returning (Powershell/VBS):
"The user name or password is incorrect."

Problem DCs (Powershell/VBS):
System.DirectoryServices.Protocols.LdapException: The  operation was aborted because the client side timeout limit was exceeded. 
"This operation returned because the timeout period expired.

This only seems to occur when you enter an incorrect password for an account and the error takes 30 seconds to return, where as the working DCs will return the invalid password attempt immediately.

Any ideas what could be causing this?

Thanks!