Lengthy hill.... under load your turbo (if you have one) will be working quite a lot, that and your tranny. Keep in mind tranny fluid is annoying, and depending on how hot it is, it expands a LOT.
There are a lot of things it can be, just start looking for leaks. Also you didn't mention if it was overheating at all, as it could be that, but I'm thinking it's the tranny :unsure:
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