1999 Mercury Sable LS

I have a 1999 Mercury Sable LS. No one can seem to diagnose my car's problem. It has 158,000 miles.
Engine is a 24V DOHC V-6. The problem originally was, when I would accelerate quickly or accelerate on a ramp up hill the car would stutter. It felt as though the transmission was slipping. That has been ruled out. Now the car does it all of the time. Level, uphill, fast, slow. It doesn't matter. Also, my gas mileage is getting worse all the time. Help.I have taken it to a shop on several occasions and they cannot figure it out. Thanks for your assistance.