Exchange 5.5 drops msgs between servers

We have 3 exchange servers, one acts as a gateway with couple of other services running, no message store. The other two are message stores with recipients split 1000 on one 600 on the other.

We have an intermittant problem: Messages sent with recipients from each server occasional will not deliver to the recipients on the opposite server message sent from. No error message received by users.

Any suggestions?