Very common question, putting quality bulbs in "will" help (Osram etc)
they are designed to take the voltage tolerance of 230 V +10%/−6%
(216.2 V to 253 V) that we have in the UK some of the cheepo ones are
not as good.
Also movement above will affect them as well, these days floors are so
thin, people moving about normally above you will degrade the life of
a bulb as its been slightly moved etc. If there isnt a room above
your bulbs then, this wont matter. In my case the R80's downstairs
blew quite often, Osram ones lasted a lot longer, but cost more, i did
replace some fittings for new ones as they looked tatty, this also
helped in my case. When you say you tested them, just wonder how ?
how old is the wireing etc ?