For what its worth, my '77 used to shift hard into third sometimes as well. It was different when accerlerating for a longer period of time like getting on to the expressway, over just normal city driving. I changed the fluid and filter and added a bottle of Lucas transmission fix. The difference was very noticable, in fact most of the time the shifts are fairly smooth now. I also think that I am not used to a three speed automatic in a car since my VW daily driver has a five speed auto.

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Tim "Furndog"
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1977 450sl How Cool!