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1990 Mercedes-Benz SL-Class sporadic downshift

sporadically my car will be traveling along at 60 mph and it sounds like it down shifts and will only go 40 mph. Every time I take it to the mechanic it won’t do it. I’ll leave and it will do this down shift thing. I have tried 3 mechanics and no one knows what it is. They have checked the transmission. What do you think?