I believe what you are experiencing is it is going in and out of Overdrive. Either turn overdrive off until you can reach a speed that will keep it in Overdrive, which is around 55 with a steady throttle. If you get it in OD and then you allow it to drop the speed below 55 it will usually downshift and drop out of Overdrive. This practice is not good for the tranny and tranny fluid can heat up. So when this happens either speed up or turn the overdrive OFF.
PS. I have the 1000 tranny and this is what I do.