Danny, this is a very strange behaviour that pix stops working randomly once/twice per month.. what I could recommend is to check regularly your pix performance and sort of create a check list of things to look into in the pix to start ruling out posible causes. You may also want to check your pix version code and go to cisco bugs toolkit database to see if you observe any bug reports that could match your stop functioning problem.
Monitoring pix performance
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_tech_note09186a008009491c.shtml
Syslog
I do not recommend this as this can consume a lots of system resources including cpu cycles but I only enable it for a very short period of time when I want to actually see some logs. Seting up a syslog and instruct pix to send logs to syslog server.
Rgds
Jorge
Jorge Rodriguez