Cruise Control Mystery

I have a 4.6 1996 p38 and when i bought it some months ago the cruise control wasn't working so with advice off this site i found out that the pipes had perished, so replacing them i did and away we went but a week ago i went on a long run on the motorway decided to use it pressed all the buttons and it worked fine for about 2 miles then cut out itself

if i left it for a while or stopped then started my run again it would work but only for a few miles
has anyone any idea's why it does this i did think it might be the relay where ever that is on the car thats get warm and cutting out
help pls

Andy

Check the condition of the rubber hoses including the one to the brake pedal.
Failing that it could be the relay or ecu which are located behind the dash at the drivers side (R.H. drive vehicles)behind the short walnut finisher. If you have rave with the electrical manual, there is a circuit/component checking procedure which will highlight the cause.