Check to be sure that when the "COAST/SET" switch is released the speed is the constant speed.
NOTE:
If the vehicle speed decreases to approximately 15 km/h (9 mph) below the set speed because of climbing a hill for example, it is normal for the cruise control to be cancelled. When the vehicle speed becomes low-speed limit [approximately 40 km/h (25 mph)] or less, driving at constant speed will be cancelled even if the vehicle speed does not decrease 15 km/h (9 mph) or more.
Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.