We have heard reports that when forwarding AND leaving
recursion enabled such timeouts occur.

Although it is difficult to logically understand how disabling
recursion (on the forwarding tab) would help those reports
have provided strong anecdotal evidence for this -- so he
might try it.

Another approach:  use NSlookup with varying explicit timeouts
to try each DNS involved -- watch out for caching effects at the
DNS servers -- and determine who is the "slow culprit".