I have noticed a number of factors which are related to deteriorating temper.
One is that the hive whose temper gets worse will ofter soon try to swarm:
this happens before any queen cells are found - maybe they bees aren't getting
their full ration of queen substance.
Bayvarol doesn't help temper.  Its always a lot easier to put in a hive than
to take out again.  I tried for a while putting a bunch of herbs in with the
smoker fuel, as much for my benefit as the bees.  Rosemary smelt fine and
caused no problem but sage caused a boil up of bees, many attacking the
smoker.  I shan't try that one again.
Chris Slade