Bucking, or "trailer hitching" as it's sometimes called, can be caused by intermittent sticking of EGR valve. Read that recently in an old post when searching for DPFE sensor IIRC.
98 Mystique LS v6 atx 110,000km
97 Contour LX v6 atx 125,000km
stock CD on LS, cass on LX
spoiler, tinted windows on LS only
4 wheel discs on LX only
All other available options on both
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