First and foremost, clean the engine and chassis ground connections (particularly the ones in engine compartment) followed by an ECU reset. If that fixes it, then that's great.
If not, test the wires coming to the coils for consistency. They shouldn't have bad connections at the other end. Plus there should be no arching from plug towers or a wire itself.
If this checks out to be OK then one or more coils could be on their way out. When coils break down, they do it gradually. If there is a hairline crack in one of the coil windings then it will misbehave inconsistently, like you said. Plus, spark will become weak intermittently, causing low torque at low rpms. Test with a different coil set to establish this.
Another less likely, but possible reason for intermittent hard starting is a bad or worn out Camshaft Position Sensor or its connector having corrosion. Also, if fuel filter is clogged up and the car is low on fuel, it will sometimes refuse to start in that situation as well.
