First of all replace the plugs. Buy the ones recommend in your car's service manual.
Then go to a very dark place and leave the car idling, see if the wires are arcing anywhere or if the coil is arcing.
If the cup and rotor have done 50k km, replace them anyways.
Also check the coil's resistance at the two metal teeth where the grip connects.
Measure resistance of the plug wires also.
Lastly drive your car hard for a couple of minutes, hard means (red line rpm/2)