HELP with Braun Swingaway

I have an older Braun Swingaway lift in my van with a 2 battery setup. It just started that when I run the lift up and down from any of the 3 sets of switched with the engine running my engine will stumble and almost die out. I had both batteries and the alternator/charging system tested and they are O.K. Any one ever have this problem? What did you find wrong?

Thanks

HELP with Braun Swingaway