Down shifting

It will downshift by itself at cruising speeds. This only occurs when it has reached operating temperature when it is cold it shifts fine. The transmission was rebuilt 1 year ago this problem started within the last six months. I had trans oil and filter changed with no difference. I took the car to Aamco transmission shop but the problem is intermittent and they couldn't get it to repeat. The tried rebuilding the valve body but I still have the problem. They checked the transmission out thoroughly all pressures and checks being good. The problem seems to occur more often down shifting at 65 mph as well as 25 mph. It normally will not during acceleration only when cruising at about 2000 rpms or on deceleration. By the way when the pan was pulled off by Aamco, the pan was clean and the fluid was clear and not at all burnt.