I've just had and solved a misfire in record time. Number 5 wire was touching that y pipe who's function I'm not sure of. It had destroyed the insulation on the plug wire.

I would start cheap and see if your spark is escaping. Run the engine in the dark look for arcing. The benefit is that you know exactly where to look.

Swap the lead and plug to a different position. See if the problem follows the bits.

I would take a look at the fuel trims. It sounds like the #8 injector may be sticking open slightly. If it is sticking, the fuel trims on that bank should be in the negative significantly. (significantly is more than 10%) This kind of problem can be tricky to diagnose. Even if you use a scope, you need to pay really close attention to the pintle hump when the injector shuts down.

The only way to really be certain of a problem like this is to do an injector balance test, but not everybody has the equipment to do that kind of thing at home. I think OTC makes a tool for this, if you want to look into it.

Anyway, to sum it up, it looks like you've more than likely got a sticky injector. There are a lot of things that can cause this, so take your time with diagnosis.