i been facing this problem with Asterisk certified/13.18-cert2 .
Server runs normal and at not specific time with no utilization on the hardware ( load average of 0.05 ) and memory at 10% utilization the asterisk starts giving problems.
The phones starts giving fake latency and getting unreachable for that matter.
This is, phone sent invite and get registered, less than a minute later it shows latency over 6000ms and of course phone goes unreachable and then in less than 1 minute becomes reachable again and this repeats and repeats, plus it starts having hung calls at the same time until we have to restart the asterisk service and all gets normal again.
Does anybody has any idea what can this be ? or maybe you faced before ?