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Have the batteries load tested. This has all the earmarks of batteries that are failing. If they are over 3 years old that is almost a certainty. Slides take a lot of power and when batterie fail they may have a good surface voltage reading, but can't deliver that at full amperage as rated. Most places that sell batterie can load test them for you. If you take them out, but sure you mark the cables and take a photo or two so you get them hooked up back up correctly. Be sure you clean the cables, especially the ground lead to the chassis. One way to tell is to take a DC voltage reading AT the motor when someone hits the switch. If it drops quickly, it is either the batteries or the motor. Do the batteries first. Get back to me if we need to dig deeper into this as all follow ups are included in our fee.
Just checking back in to see if you perhaps missed my earlier response on your question? And if you got this resolved or do you still need some additional input from me here?