98 pontiac grand prix gt?

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my grand prix shifts hard,bucks at lower speeds and bogs down on take off after it has warmed up. it feels almost like it is being forced into gear once i get going. trans checked out. no ses codes. electrical checks out so far. belts, plugs and wires replaced. what else can it be?? this has been a continuing problem over the last year that has not gotten worse, thank goodness. please help