Coils are not cheap. Even modules are well priced.
Definately pays to check that the module is well earthed. Replace the rivet with a bolt. Clean the contact surfaces.
Check that the HT lead is well insulated. I have had the power torque coils that have let water in where the lead goes into the coil. This causes intermittent spark issues.
Never trust a spark plug. Buy another, having spares around is not a bad thing and they are not expensive. I would also suggest an NGK plug rather than a CJ8.
