Erratic firing of cylinder. If the wire is good, the signs present themselves at the plug with lots of carbon, after some urban driving. Looking at the coil pak might show some small cracks on/near the corners which seem to be the weakest spots. Sometimes you can even see tiny sparks escape the pak if you can find a spot dark enough to observe. All of these problems could worsen in damp/wet driving.
Fastest way to t-shoot the prob is with a scope that can monitor each wire's throughput while the car is running.
The paks can cost a lot depending on whether it's a used one($20-$50) or a new one,($100's).
Try a wire change first.