The car is a 1997 406 - 4 cylinder petrol engined auto purchased new,
now done 270,000 km without any serious issues and no previous
problems with cruise control.

Almost all operation of the car has been on motorway under cruise
control.

When started from cold, (daytime temperature 20 degrees Celsius)
cruise control can be engaged normally but will invariably stop
operating after 3 to 4 minutes (less if the temperature is higher) and
can not be restarted immediately.

If I continue driving at motorway speed limit of 100 kph occasionally
attempting to restart cruise, I usually succeed after about 30 minutes
but operation will stop after only 1 to 2 minutes.

When daytime temperatures exceed 30 degrees Celsius, cruise will not
work at all.

I have driven the car this way in the two weeks since the problem
first appeared and have found it to be quite predictable, i.e. it
never operates longer than about 4 minutes, might be restarted after a
half hours spell but will then only run for a minute or two and will
not operate at all when hot.

Car spent most of yesterday with Peugeot dealer who replaced brake
pedal switch and steering column cruise control switch as well as
checking all the wiring. None of this has had any effect, the problem
remains. I have tested it today on the freeway and it behaves exactly
as before.  

Dealer, who is not charging me for his unsuccessful repair attempt,
says I should talk to an auto electrical specialist.

I suspect the vacuum pump. Any suggestions?