We’re been using it since v3 and are currently on v9. Our gateway runs on it’s own machine (win2k) while the domino sever (8.5) runs on ReadHat Linux x86. Even now the SMTP gateway is a bit pants and tends to fall over now and then, but can say I don’t recall it doing anything like what you are experiencing.
It sounds more like a domino problem though than a faxpress one. How have you configured domino to send to the gateway ? And is it running on the same machine as domino ?