So, I've got a 2010 Nissan Titan SE with 105k on the clock. I bought it two months ago from a Ford dealership who told me it was an older gentlemen that had traded it in on a new F-150, and that he had JUST put a "Brand New" Trans in the titan before trade-in, which seemed odd but okay, anyways after about 1100 miles and two months of driving, here's the issue:

The transmission overheats really bad in all conditions except while moving at highway speeds, and the trans temp gauge regularly reaches the "H" on the high end which I fear is doing damage and burning up the trans. Honestly it leads me to not believe that dealership at all that its a new trans as I feel a brand new trans shouldn't overheat from regular driving.
Literally if the truck sits idling in traffic at all for any amount of time, it overheats and I live in SW Florida on the Gulf Coast where its VERY hot 24/7 so obviously this is an issue.

Now I do have a full powertrain warranty on the truck for the first two years of ownership but its only with THAT specific ford dealership, and obviously a Ford Mechanic isn't going to know too much about Nissans, and I'd much rather have a qualified Nissan Tech taking a look at it but I don't wanna pay for that when I have a powertrain warranty, yanno? So Im wondering if I should go to a trans shop and have the tranny flushed then see if that makes any difference before I let the dealership have it.

Help please, any suggestions or advice would be highly appreciated, Thanks in advance!