More then likely this is due to the brake pedal position sensor not functioning properly. You do not have a brake pedal switch meaning it is on or off. You have a sensor so it gives the control module a resistance reading based on how much the pedal is being depressed.
When the brake pedal is depressed the cruise will turn off. So the fact that the system activates shows that it works but it will not set because it thinks the pedal is being depressed. The times that it does work but then fails, is the sensor working and then it starts reading a resistance and it thinks you are slowing/lightly pressing the pedal so it shuts it off.
I attached the process below on how to replace this sensor. Let us know what questions you have. Thanks
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Sunday, December 27th, 2020 AT 3:26 PM