Could be many things.

Start by checking the fuel and electrical systems.

Fuel: Is there sufficient pressure and flow?

Electrical: When was the car last tuned up? Cap, rotor, wires, plugs....

I'd look at the electrics first. A poorly maintained system can cause your symptoms.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles