My gf has a 97 mercury sable with 140K miles on it and for some reason the car shakes eratically. Its not a hard shake but its just enough for you to be like WTF? When the car is in Park it idles at 1k. But shakes a bit. But when u put it into R, or D, the RPM drops to 500. When u drive the car it stutters. It'll go but it will seem like it stutters. I checked all the fluids, seems to be ok. Also, when you turn the HVAC on, the shaking is even worse. Has this ever happened to anyone before? PLEASE LET ME KNOW. My gf needs her car for school and the sooner i know what the prob is the sooner i can fix it or have it looked at. BTW, the SERVICE ENGINE LIGHT is on too. After its on for a while it wil start to blink. THANKS