My new 2016 PM 136 low roof gas keeps down shifting on the highway @65+ with or without cruise control. No check engine light, shifts @ 10-15 second intervals, up and down. Horrible gas mileage. Dealer can't figure it out yet, only 400 miles.
you don't mention how heavy you're loaded, or how much wind your fighting when this happens. If you are running a heavy load you may want to consider the tow haul button. Mine will drop in and out depending on grade and load, but not to the point I would consider it a problem. I do notice it more pronounced over 65mph
If the transmission is hunting between gears the Tow-Haul button can help as mentioned above. I find that I can solve the problem on long mountain grades by moving the shifter to the left then manually shifting down one gear. Yes, it raises RPM and uses a bit more gas. Gear hunting is usually caused by the computer being on the verge of shifting and very slight needs of more power cause the shift. Slightly changing speed may help. Finally, the transmission/computer will sometimes "learn" your driving habits and adjust. Maybe you should see what happens at 1000 miles.
Thanks for the responses. The PM was not loaded at all when we first got it. Drove it from PHX. to Yuma, AZ , 186 miles. Didn't matter what speed 65 to 80 and shifted constantly up and down 10 to 15 second intervals and with tow/haul on or off, cruise on or off. Pretty flat across the desert. I was thinking the computer or programming?