1999 Mercury Cougar

I just bought a mercury cougar -99, had 2 mechanics check her out before both telling me she was a good car to buy and nothing major going on with the car exept a small oil leakage.
2 weeks later, check engine light comes on, mechanic again says its nothing to worry about she only needs a censor for something.(Sorry am a bad car person and its all greek to me when they talk). The day after she starts shaking at every red light that I had to stop at, and this morning I turned the ignition she started and then went dead 3 times she did the same thing, I finally got her going and was able to get to work. Now she is fine as long as I step on the gas pedal but as soon as I start breaking she starts shaking and start to run out of steam so to speak.