Is it the same gear failing in every box. One of the down stream clutches could be stalling and causing a backlash. I would suspect the ac unit gear first. I would shorten the oil change interval to 500 hrs or possible 250 and pull an oil sample for uoa. Short of drilling a hole in the case for a thermocouple, I would get some of the temp sending wires and epoxy one to the the side of the case. OTC makes or used to make a handheld temp tester that had two terminals on it. You hard wired what you wanted to monitor and then just walk up and plug in the lead. We use them at our plant for some of the hard to reach stuff. What does the oil look like after 50 hrs?
