oconto

I have an auto trans. that I changed the fluid on after cleaning system with additive so the system is fresh with fluids (nissan products). When I am going on a flat surface with no load and no wind the truck will kick out of overdrive and then back into overdrive. I think it is the torque converter locking and unlocking as it doesn't seem to be dropping a gear. Is seems when the truck is good and warm (twenty miles) that this sympton goes away or at least drops way off. I have no idea where to start?
Thanks!