This could be a couple things. The vehicle speed sensor, your brake light switch or the ECM.
Start by looking for a loose connector on the front of the cruise control actuator. Try to unplug it and plug it back in. Sometimes it works??? If it does let us know.
or
Is it still under warranty? If it is, try to go to authorized service center near you for diagnoses and repair, these could be replace without any cost on part and labor if is still under warranty period. Cruise control is an electronic device and can be only diagnose by a scanner. I have research this
http://bit.ly/14Ms2U2 for you but it is very technical thought if you understand.