Automatic transmission disengages at 40 mph

Transmission distinguishes at 40 mph reengages at 35 mph, can get faster if I make it down shift. My car has a 2.2L engine and the 4T45E transmission shifts like it's supposed to through all gears, but it won't stay engaged above 40 mph and slows back down to 35 mph, then reengages back up to 40. No obvious leaks. Bought the car from a neighbor who had taken very good care of it.