Chevrolet Repair Questions? Ask a Mechanic for Answers ASAP
I have seen this concern multiple times and there might be two things happening:
The supports struts have leaked out the hydraulic fluid and are no longer helping the gate open which is triggering a pinch condition on the motor. That means that the door thinks it is hitting something and reversing the motor, but in reality it's just too much weight on the motor without the help of the hydraulic struts. The first thing to do it replace the struts, especially if the lift gate seems like it will fall back down on its own once opened. You don't want that thing coming down on your head. They are really easy to change.
The second thing that might be happening is the gate latch is malfunctioning. The latch has an assembly of pawls, sectors, and a latch that work together to tell where the door is in the close cycle. When the latch gives out an incorrect signal the system freaks out and the door will open, close, shudder, and then the chime on the dash sounds a couple of times. The door then usually closes partially on it's own and stops and sometimes the tail lights flash. -is this happening on your car?
I can look up a procedure on changing the lift struts if you would like to tackle them yourself. You will need a helper to do it. It's not a one person job.