It is more likely to be the ignition module than the spark plug. A bad spark plug mostly makes it hard to start, sluggish when revving up, and reduces maximum power. However one of the tests for a bad ignition module is checking the spark at a known good spark plug when the module is hot and has been running long enough to give trouble.
Bear in mind that you can have trouble with bad module earthing - it isn't always a bad module.
