Transmission stutters when changing gears usually in higher mph range. on 1998 Mercury Grand Marquis

I have a 1998m Grand Marquis gs. The transmission stutters when it changes gears every once in a while. It makes the whole car vibrate, and seems like your driving on a rumble strip usually at the 50 mph range. It gets worse when the AC is on. It doesn't do it so bad when i manually turn overdrive off. No lights are illuminated on the dash...

By describing it as feeling like a rumble strip, I can definitely say that this is a classic torque converter shudder.

Change the fluid and filter. All the fluid. Use a fluid exchange machine if necessary. Make sure to use Mercon V. Give it 100 miles or so - and hopefully the shudder will dissipate.

If it does not, it almost certainly will need a torque converter replacement. See a qualified transmission shop or the dealer to get a good diagnosis.