RPM's needle bounces and car overheats on long trips using air conditioner

Started out on a trip from Vegas to Kansas City, in late July 2016, two hours away from Vegas rpm's needle fluctuating back and forth between the two and three. Car kind of chugs a tiny bit but you can feel it. Stopped at automotive shop he put it in diagnose says " cannot find a thing " it did this fluctuating all the way to Kansas City. Went to another automotive shop. Told me the same thing. Headed back to Vegas after several weeks. Made it to Colorado stayed a week. Left in mid August headed back to Vegas, after about seven hours or so (air conditioners on high) rpm's starts fluctuating all the way to five and six or seven, and now temperature gauge is in H. Turned off AC drove rest of way to Vegas (106 degree temperature outside, ) but made it home. It stopped doing the bouncing rpm on high numbers, and temp went back to center. I lost confidence in automotive shops so I drive it around still in town six weeks later no problems. But took it to Nissan dealership, he says it is transmission. Not sure, like I said lost confidence. What are your thoughts?