1999 Nissan Maxiam Problem.....

I have a 1999 Nissan maxima with 60k. I took my car to Bert Ogden Nissan dealership in McAllen TX. because the ?service engine soon? light came on and my car was not performing well. It drives smooth but every time it is in a complete stop or on park, the car would start to go FORWARD or SHAKE. They said that it needs all six engine coil, so I did pay $650.00. When I took it home, the car started to do the same think and then the light went on again. As of right now, it have been almost two weeks that they can not find the problem. They have replaced the harness and the computers but it is still the same. I?m very up sad and don?t know what to do. If any one can help me out, I really appreciate your help and your info.