may cruise control is hard to set sometimes it is set and it just goes off

I'm assuming you don't use the cruise under 30 MPH...and don't rest your foot on the brake pedal. Your problem may be in the brake light switch circuit. Have the switch checked behind the brake pedal. (This may sound odd, but on the HHR's if more than 2 of the five high mounted rear brake lights are out, the cruise will act up.) Clare