I have a 2001 Disco II V8 and have been troubleshooting the cruise control for more than 6 months. I have replaced the ECU, actuator, dash switch, and steering wheel switch - nothing worked. I read somewhere that it may have something to do with the radio. So, I got in the truck and went for a drive, turned the radio off, activated cruise control and the cruise control engaged and held a steady speed. At this point, shaking my head, I turned on the radio and all was good for about 2 seconds. When the truck tried to slightly increase its speed, the cruise control shut off. I repeated this a few more times and the same thing kept happening. I am thinking the cruise control and radio share the same ground or relay or something to do with the steering wheel controls. Does anyone have any suggestions?

 

Leave the radio off.

Try to increase the speed via the steering wheel paddles.
one will increase the speed 1 mph per press.
See if that works.

Also if you keep your left foot on or near the brake pedal, you may be reflexively tapping the brake.

If you find the cruise really does get affected by the radio, then try to ground the radio with another ground from the radio medal case to a ground on some steel under the dash.

If this radio is an AFTERMARKET which was HACKED in there then
trace the power wire for the radio - may have been spliced in at the cruise control fuse.

And speaking of fuse - replace the cruise control fuse anyway - fuses do wear out from thermal expansion and contraction.

Sockets get dirty and corroded.