The CC is set up so that it kicks off if the speed drops 10mph or so below the set point (truck can't maintain speed in that gear). Is that what's happening? Owners manual has the details.

Make sure the actuator is pushing the pedal all the way down when it needs to. If the cable is too loose or something the actuator might not be able to put the pedal to the metal.

'97 4Runner, '06 F350, '86 4Runner, '05 WR450
http://home.4x4wire.com/erik