Improper toe-in front or rear will make the car twitchy. Tires with different tread designs front and rear can make the car practically uncontrollable on some roads, as some tread designs react differently to different road contours. As jbajoo7 said, worn subframe bushings can cause the problem... play in the steering gear or worn components anywhere in the steering can cause handling problems.
Where to start??

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C