Just by your description (detailed) id say it could go a few directions ??4 a cause, one way is the 40 mph is near the speed that overdrive is engaged (42 mph) these do sometimes have problems inside them (gears ) also the stutter you mention is sort of an indication that your torque converter didn't lock up or is locking/.unlocking this is an indication of a immanent torque converter failure , if car was made past 2001 then chances are good it has some kind of transmission control system that could log a fault to help you look for a cause like a faulty speed sensor or something , ck your codes !!
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