2006 Nissan Maxima shifting issues

Having a problem with my Maxima. This started on a trip to Colorado and I thought it was a high altitude issue. While driving up into the mountains we started having problems with the vehicle shifting out of gear. We turned around and when we stopped at a stoplight it was very sluggish taking off. We stopped and checked a few things. Fluid looks normal, Air filter was pretty dirty, we banged it out and replaced it when we got to an auto parts store. Car ran fine the rest of the trip. Now we are having the same issues back in our low altitude home town. We tried pulling the codes, but apparently there are some issues with the power the port? We have one power port inside the car that has never worked and we were told that sometimes that's on the same circuit? I dunno. Wife says she thinks it's sticking in third gear, which would explain the sluggish take off at stop lights. Thoughts?