At this point you need to pull all the spark plugs out, keep them in order that they were removed and look at them. Are some black and some clean? If so...the injectors would be a problem.
If you have a scanner that gives live data, look at the fuel trims and the injector pulse width.First look to see if they are both pretty close to each other.
A fuel trim that is going - > 5% ( like - 25% for instance) on one bank that means the PCM is trying to correct for a rich running problem limited to one side of the engine.
Most likely leaky injectors.
If the trim is going + > 5% ( Like +25% ) that means it is adding fuel to correct for a lean condition , like an intake leak for instance. Trims run from data supplied by the oxygen sensors, so if trims are moving, sensors are working.
Cylinders 6 and 8 burning plug wires may be indicative of excessive heat, from excessive fuel, caused by leaky injectors. Just sayin. Check it out post back here. LA Tech
Fact is stranger than Fiction