Mercury Grand Marquis car shudders at low RPM?

Around 40 to 45 is when the tranny will shift into over drive, OD, driving normal. There is a little push button on the very end of the gear selector arm that will turn the OD off, & it's there just for that reason. If you accelerate normally up to 45, the tranny will work thru the gears until it shifts into OD. Then, if you keep that 45 mph speed, +/-, the tranny tends to shift down to drive, D, then back-up to OD, as the load on the engine changes. To find out if that's what is happening, turn the OD off with the button - A OD light will show on the dash when the OD is OFF, then drive normally up the 45 or so, & stay there. If this shutter is gone, then that's what was happening. Normal driving leave the OD on, then you don't have to worry about it. But, if you're on a 45 mph limit road & the car can't decide between the tow gears, turn the OD of, until the speed limit changes. If you turn the car OFF with the OD OFF, it will reset to OD ON ( no light ) when you start it up again.