Ignition leads are known to break down (ie fall apart) where they connect to the plug and also the carbon ribbon in the leads break, causing the spark to have to jump the gaps as it were. This is worse under load. The cost of a set of leads is minimal and the results can transform the running and fuel economy. If you do change the leads and plugs, make sure that you vacuum out any debris around the plug base before removing them (or risk the stuff going into the cylinder!). I used a 25mm plastic tube taped onto my normal vacuum hose, over the plug. A fair bit of broken plug cap could be seen in my vac's container at the end of the job!
Ignition components.