Having problems with my cruise control. After replacing the vacuum line, it started working, but know only works intermittently. I can occasionally turn the switch on and off and get it to work, but this is only seldom.
Steve, make sure you check all the hoses that come off the t connection under the hood. if one hose is spliting or cracking than its a hihg probability that the rest of the vacumm hoses are to. the other thing to check is the actuator control that is on the back part of the breake pedal.
get your head under the dash and check the back side of the brake pedal. there will be a mechanism with a black plastic plunger. this mechanism like to dislodge itself or twist a bit, so that sometimes it will work sometimes it wont depending on how it has lodged itself.
I had the EXACT problem in a 97' Disco, and I worked it out like you. When going above 30/40 mph you can engage. Reset the switch on the dash and give the reset (-) switch on the steering wheel a few pumps, you should be able to hear the relay behind the glove box click with every cycle of the switch. When you hit the SET (+) the relay should click one and cruise control is activated. Let it go for a while until it fails again, try to reset and listen for the glove box clicking, (probably won't work). I could reset on the dash and sometimes get it back...it turned out to be the Cruise Control Module (don't know the full term) located behind the dash. This is over 200.00 from the dealer but I got one from ebay for 80.00 (used) from Roverland inc. These guys back up their products and if its not listed they can usually find it. (The other thing may be the speed transducer, mine got a little dirtly on the connections and when the speedometer jumped the cruise kick off.