cruise control module
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.