the automatic transmission sounds like its flaring. thats a common problem are there are probably (literally) hundreds of threads about it. it could take you days to read through all of them! the shifting quality (hard/softness) of the shifts on these cars is controlled by vacuum. if the shifts are too soft it will "flare" between shifts (take a long time).

running out of power could really just be a result of your elevation and driving a non-turbo car. how badly does it bog down? for example, at 6000ft and a steep incline, i probably wouldn't expect you to top 50mph, especially if its in 4th.

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1981 NA 300D 310k miles