My 1999 Mercury Sable shutters between 30-45 mph.?

Transmission checked on at transmission shop machine, okay, put on auto mechanic machine nothing showing wrong no code errors. Had trany flushed and added Lucas, had Ford reflash fuel injectors, mass air flow had been changed and was checked again, coil replacement checked....okay $500.00 and still the same! Any further ideas?????? It has 105000 miles on it. Runs like a top except for the 30-45 mile frame and trying to keep it in that range. Below 30 is okay above 45 is okay...