Maybe its more of a throttle position to speed issue? with the smarty or other tuner that increases power, less TP is required to gain MPH. Maybe its causing the tranny to go nuts? i dunno. My 05 does the exact same thing as you describe. Stock is great. load a tune here comes the stupid shifts. I switched to a juice attitude set up (something that doest effect ECM at all) and problem went away. My superchips flashpaq and smarty jr both drove the tranny nuts. Just my experience tho.