Hi. Welcome to Just Answer.
This is usually caused by a sensor light going out or simply a loose connection somewhere.
Check both lines going from the sensors to the main unit. If they are ok and the connections are tight then suspect one of them going out.
There are generic replacement sensor light kits available for a reasonable price at places like Home Depot, Lowes, Sears, and most hardware stores.
Unfortunately there isnt a test to prove that the lights are the prtoblem.
It has been my experience that when a door starts to act like this that it is the sensors 98% of the time.
Please let me know if you need anything else.