Low power

If your engine is overheating, the engine computer will purposely shift gears rather quickly putting it in a high gear to keep the engine RPM down This helps protect the engine from damage due to overheat condition. It will tend to keep it in overdrive or a high gear even when normally you step down on the accelerator and it would kick down a gear or two. It won't do the normal gear change when running real hot. If you step down far enough it will downshift. Although should be avoided and those symptoms above should alert you of a problem, possibly a major one if you continue to drive it.
Hope this helps. I had experienced this on two trucks a Ranger and a Dakota. Buddy of mine worked at Ford told me this info.