Yesterday, I got in my 1980 300SD and drove it as usual (It had been sitting for about a week). It shifted between 1st through 3rd just fine, however, at about 40mph, it suddenly down shifted, then a second later shifted to third again. It does this consistently. I noticed something really strange as well, as I pressed the accelerator, the speedometer needle would behave like a tachometer, where it would move up the speedometer in tune with my foot. I checked the fluid and it's clean and appears to be at the correct level (I checked it with the engine running and in park). If I accelerate quickly, it'll get to third and stay there fine. Am I looking at a lot of $$$$ here or is it possible an adjustment of some type would work? The car has 353k miles on it and I've never had one once of tranny trouble with it until now.

Thanks,

Paul

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1980 300SD (352K)
1978 Chevy Nova (30K original miles)
1994 Dodge Caravan (146K) 4 cyl - why?
1999 Toyota Corolla (Neo synthetic only please)