We had the same problem with our NH4000 and let it go till we ended up having to replace linear guides, pneumatic cylinder, linear rail, and all proximity switches. What the problem is is the internal buffer in the cylinder has failed and you cannot rebuild it. You will need to get a new cylinder. If you let it go it will damage your linear guide bearings and then the rail causing the door to droop and then start to chop up your proximity switches which will give you alarms that the door is not closed or not open. I know this from running our machine balls to the wall and owner not wanting to stop and address situation till machine just stopped and I rebuilt whole ATC door assembly. Fix it now before more problems arise.