Lost calls

Ive been running the freepbx distro with a FXO card for a couple of months now. About every 3-4 weeks the system will stop processing calls. The calls will ring through and never even touch the voicemail. It starts with FXO line one and then a day later it affects FXO 2. The problem is resolved if the system is restarted and works fine for another few weeks. I have tried looking through the logs in th past but have not found anything useful. I do not have the logs to post at the moment. Has anyone had this problem or know anything about it?

What versions of FreePBX, Asterisk and DAHDI? Distro install or hand built? What brand and model of FXO card?