2003 Mercury Sable buck from stop unless let off gas or sto

Firstly, do you get a check engine light? If yes, I'll need to know what codes are stored in ECM memory. Then, spark plugs can cause this. When was your last tune up? Please advise.

I re-read your post and what I see is; The "bucking" you described sounds like what is called a "lean cough". This is when your cruising down the road, top down, radio playing and your computer is doing just what it's supposed to do. It leans out the air/fuel mixture for optimum economy. But, you need to change lanes so you accelerate just a little bit and "poof" the engine coughs. If I am correct so far, the cause is; the ECM can't richen up the mixture fast enough to compensate for the added air allowed by the throttle. Two sensors can cause this condition. 1- throttle position sensor TPS). If you do a lot of highway driving, it will have a tendancy to wear a "weak" spot in the sensor's coil 'cause that's where you hold the throttle 80% of the time.
2- The O2 sensor. The signal from this may be holding the fuel/air mixture a little "too" lean for the ECM to compensate for light acceleration. Each of these sensors could be defective but, still be in range enough to not trigger the check engine light. My first suspect would be the TPS. Hope this helps.
Note; Both these sensors can be tested with a volt/ohm meter or a hand held diagnostic unit.

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