2000 Mystique hesitating and cutting out

Hi all,
On Saturday night, I was driving home in my 2000 Mercury Mystique when I noticed my engine beginning to hesitate whenever it was just about
to change gears. After a minute or so of this, I could not accelerate beyond 40 MPH. Then, as I put on the brakes for a red light, the engine died. Cranking it started the engine again (after a while of cranking), but it would only work until the next time I had to apply the brakes or accelerate abnormally. I limped it into my home mechanic, who told me the next day that I had misfiring cylinders. He replaced a couple of parts and told me that I needed to get a new fuel rail sensor and O2 sensor the next time the Check Engine light came on (according to his computer scanner), but now Iíve found that the car is having the same problems every time Iíve been driving it for a while (hesitating and stalling). I took it to the mechanic again, but he insists that the parts do not need to be replaced yet. In the meantime, Iím afraid to drive more than a few miles away from home. Any thoughts?
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