Steps to reproduce:
1. Open hardware template (for example Brocade FC SNMP)
2. See In Temperature discovery items
sensor.temp.valueswSensorValue.{#SNMPINDEX}
sensor.temp.statusswSensorStatus.{#SNMPINDEX}
with delay 3m

and trigger with expression like
avg(<item>,5m)>{$TEMP_CRIT:""}

Result:
Triggers calculated average for two items values. Also trigger's expressions calculate every 30 sec, so in some cases only one item value can be present for 5 minute  and triggers can blink.

Expected:
Triggers fired when average value for some items values more than threshold.

Some of affected templates:
Brocade_Foundry Nonstackable SNMP
Brocade FC SNMP
Alcatel Timetra TiMOS SNMP
Cisco CISCO-ENVMON-MIB SNMP
Dell Force S-Series SNMP