Change the plugs for the correct ones.

A lot of people do not realise how important it is to have a plug with the correct heat range fitted. If the plug over heats then the earth electrode can start to glow. This is a hot spot in the combustion chamber and can ignite any fuel which comes into contact before it is due to be ignited by the spark. Det occours and good bye pistons.

Once you get the plugs in, see what it drives like. It is possible that the plugs have been set too wide and the spark is being blown out or it may have fueling issues..