You would have a missfire for these basic reasons.
- Spark Plug
- Plug wire faulty or grounding out
- Injector plugged or faulty
- Coil pack faulty
- Intake manifold leaking air/coolant into the cylinder
- Head gasket leaking
Check your coolant level and make sure it's OK. Obviously you have already replaced the plug and coil pack so you can check out the other options.