1991 Ford Tempo Transmisson down shift's after getting driv

Assuming the fluid level is ok the first thing I would check is the kickdown linkage which is a rod that goes from the air intake throttle butterfly down to trans. It's under the plastic cover on windshield side of throttle butterfly assembly. Cover held on by 2 bolts. Ford used a plastic bushing to attach rod to trans control arm and this breaks and can allow rod to detach from trans control arm and cause shift problems. Generally speaking transmissions shift later when cold and sooner when warm. In your case it is so much sooner that a downshift may occur when warm.