Brake problems on '02 Santa Fe

I have an 02 Santa Fe, 2 litre turbo-diesel 4wd with a weird brake
problem. Every so often, very infrequently (maybe once a month or
less) I put my foot on the brake pedal and the pedal feels rock hard
and there is very little braking effort. The car does eventually slow
down but its scary. After a few seconds, the normal brake response
returns.
I bought the car second hand about 8 months ago and when the problem
occurred initially I sort of dismissed it as being a figment of my
imagination or something but no more. It definitely happens !
It feels to me like the brake servo is sort of going "off-line" for a
few seconds and then coming back to life again. The car has been
thoroughly checked by the local Hyundai main dealer who have found
nothing wrong and declared the car perfectly safe. Fortunately, the
dealer I bought it from (not as it happens a Hyundai dealer) have
accepted that there is a problem and have taken the car from me for
further investigation. Has anyone any come across this kind of
problem or got any suggestions as to fruitful avenues to explore !
One other piece of information that may or may not be relevant is that
as far as I can tell I've only noticed the problem at low speeds say
less than 25kph or so, often much less.