1993 Thunderbird Overdrive?

I can drive in overdrive until i get up to about 40mph and after that, the engine revs. I was thinking it might be electrical because sometimes I can get up to 60, and sometimes it goes out before 40. Does anyone know what this could be?

Details: 1993 Ford Thunderbird LX V8

Check your fluid level… It is supposed to be checked with the engine running, otherwise it will look higher on the dipstick.

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