Have you noticed any fuel leaks? Wen I had a similiar problem it was due to the fact that my IP was leaking; of course my engine wasn't getting enough fuel. Definitely sounds like a fuel issue. How steep is the incline?

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1995 E420 152k
1991 300D 2.5 348116 daily driver
1992 300E 2.6 196k-work in progress(head gasket)
2002 BMW 745Li 93k