Are you using freeswitch to send & receive faxes? I've been using it for almost 6 months now and it feels much more robust than asterisk's system.
I have a small 50 line ruby script that converts attachments from emails into outgoing faxes, and a 10 line ruby script that converts incoming faxes into emails with a pdf attachment.
You should try freeswitch instead -- the rxfax thing from Asterisk would sometimes truncate incoming faxes. Not sure why.