Fish tail
 

I have a 89 190E model 2.6 with 157K on it. Whenever I step on the gas pedal (around 65 mph and above) the rear end swings left and right for few time and stop, this would not happen when I am driving alone but when there are passengers in the back.
It has Potenzia 205/55ZR15 tires, the rear has regular shock observers with coil spring between lower arm and the frame - No hydraulic or air suspension on this model. Any idea? Thanks:confused:

At your mileage there's a good chance you have a deteriorated bushing in a rear suspension link. If you get the car up in the air and vigorously shake the links, you will probably find one or more loose.

This appears to be a common wear item in your mileage range, and there have been a couple of similar threads in the last week or two on this same anomaly.

Duke