Can you check vaccum on intake manifold somewhere, it should be between 16 and 22 inch Hg at idle. If it is too low at idle, there is a problem in the exahust system, most likely a clogged catalytic converter.

Fuel pump possible but is less likely given that you can get the car to get to say 4000 rpm and keep it there without a problem.

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Saumil S. Patel