Most references I can find are analogue card related. In preparation fro the next failure you might want to do the following:---
You might want to turn on full logging to see what is happening just before the freeze... In /etc/asterisk/logger, uncomment the line which reads
;full => notice,warning,error,debug,verbose
Also, when it freezes, capture the output from dmesg and publish the last few lines of dmesg and the last couple hundred lines of the full log (var/log/asterisk).
S