91 Mercury Tracer Automatic shifting problem

My daughter has a 91 Mercury Tracer which she uses for college. It started 2 weeks ago with these symptoms. The car will go into o drive (high gear) at some speed, Then it will shift every 4 seconds into
the next lower gear, Just as if you had stepped on the gas in order to shift down. The car will maintain the same shifting pattern every 4 seconds from about 30 mph to about 70 mph. The shift to the lower gear is fast and will never go completely into the lower gear. It shifts back to the higher gear.
I have not found any loose / broken / deteriated vac lines. The linkages seem to be OK. The car does not slip when shifting into any gears which would indicate a defective transmission.
Does anyone have a idea where I should be looking? Oh yes, there is no check engine light and no error codes in memory.
Thanks for some advise

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