the first step is to make sure that there are no leaks in the exhaust.
next thing would be to check the spark plug wires for arcing. Replace the distributor cap and rotor if they have been used for more than 50,000 km.
also check if the ignition coil is arcing, if not then still check it's resistance.
if ignition is good then something is wrong with fuel injection (i think it will be a problem with ignition)