Welcome to the Forum.

Two things come to mind - fuel delivery and catalytic converter.

The car could be starved for fuel, usually by an old filter that needs changing.

The ceramic monolith inside the catalytic converter can sometimes break up and restrict exhaust. Give it a kick, if it rattles then you've found your problem. A more involved diagnosis would involve disconnecting the exhaust before the cat and see if the car runs a lot better.

Both these problems would affect acceleration and high speed performance.

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95 E320 Cabriolet, 169K