I bought my used 97 SE and it had the 4 cylinder wire (and assuming plug) replaced. It also had the corresponding coil pack replaced.

Recently started throwing the P304 code, and replaced the wires and the 4 cyl plug after noticing that there was arcing going on with that plug and wire.

About a month later, I'm noticing the same symptoms happening again, but no code yet.

My guess is this motor had that problem before, and the previous owner hadn't fixed with with the coilpack/plug/wire fix.

Is there anything that would cause a specific plug and wire to fail (arcing?)

Check over the routing of that wire to make sure it'* not near or touching any other wires. It could be a coincidence, that it'* the same, but check none the less.

Depending on where the arcing is occuring may or may not give this idea merit.

From the factory there are metal shields over the boot at the end of each plug wire (by sparkplug). If the shields are missing the boots might be getting hot and brittle.

Another idea is .. quality of wires being used. If the wire isn't a lifetime wire, I've found quality is usually poor and they can be bad in less than a year.

Yeah, I still have the metal shields installed.

The last guy took real care to rout the wires as they were from factory. This time around we routed them a little differently, spaced them out more.

The wires are AC Delco, as are the plugs. I even threw out a set of Bosch plugs that came with the car in order to put the Delco plugs in.

And the wires are clearly arcing if you mist them at night? That'* pretty odd that you'd burn through wires, especially quality ones.

Any mice or critters getting in there and chewing on them?

I haven't tried misting the wires. I'll try that this weekend.

Do that when it'* dusk. If you see sparks/note a difference in the engie. That should lead you to the offending area.