I have a Performa 6400 running system 9.1 (although the same thing happened with system 9) with USB card (Interex DualPort), with a Global Village Teleport USB 56k modem. The modem disconnects at any time between immediately and 2 hours, saying 'Apple Remote Access. You have been disconnected.' in a window when it happens. After I am disconnected I cannot reconnect until the computer has been restarted. It has always done this, I sent the original back through the Global Village support and got a replacement and it still happens. Because it happens with 2 modems it's unlikely that both the modems were faulty, so it must be something else. I have tried shorter modem cables, various versions of Remote Access and Free PPP, but it still happens. I have used BT Click, AppleOnline, Madasafish, Abelgratis and currently Freeserve ISP's (I am in the UK), and none of my modem settings anywhere are set to automatically disconnect after a set period of time.
P.S. I realise that all modems do get disconnected sometimes, but I don't think what happens to mine is normal, help!