I'm facing a similar situation:
After a lot of time working without problems, some interfaces from a router (Cisco 7200 SNMP v2) went down (unplug) and went up again (plug).
After this, zabbix is not getting some snmp items from router, but the strange thing is that zabbix is getting the "in traffic" on same interfaces, but not the "out traffic" of the same interfaces.. and on other interfaces is getting the out but not the in traffic...
In zabbix web interface the items are marked as "not supported" but, from zabbix machine I can do a snmpget of that item and get those unsupported values. Trying from web interface to disable/enable again, but no success.
Looking in the logs, I found nothing.. So.. zabbix_server restart fix the problem.. but this is not a nice situation, because I can't know when zabbix is doing this kind of strange things in order to restart it...
Zabbix 1.8.2. compiled against net-snmp 126.96.36.199 (x64)
Cisco 72000 Software (C7200-IK8S-M), Version 12.2(46a)
Any advice on this??