One thing that you can do is give the truck a little bit of gas to help it decide on what gear it wants to be in. What is happening is that at part throttle the PCM tells the truck to shift at specific mph marks. If you are creeping along when you reach that speed the transmission will still shift but because you are hanging around the upshift/downshift peak it gives you the results you noted. When I approach the shift points I will throttle up enough to get the truck over the point with a few mph to spare before I go back to my feather pedal. I then hold that slightly faster speed and not drop back down into the inter-shift condition range.
I should also add that my custom tow tune is not really intended for towing. I had tailored it based off of a tow tune for off road conditions before I sent off the info to have it made. It just happened to be more stop/go traffic friendly while getting better mileage so I have stuck with it. Now when I go on trips that involve going faster than 60ish with heavier pedal I switch to the performance because it drinks fuel pretty quick at highway speeds.
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