From my understanding, reasons could be either fuel delivery (fuel filter, fuel pump, pump relais [MAS unit in your car], fuel distributor) or igniton (distributor, rotor, coil, wires, sparks, over-voltage protection relais [OVP]).
Most easyly, I would first visually inspect ignition wires, distributor, and rotor. When was the fuel filter replaced last? Could you exchange the OVP for a test? If nothing obvious, you have to search systematically. Searches in this forums may give you more specific information how to do so. Do you know a skilled mechanics? Good luck, Bruno