Safari AWD '99, spark from coil wire to housing, rough running

I had starting problems with our 1999 Safari AWD. When it finally started, it did run rough, engine shaking as if one plug wasn't firing.
Removed distributor cap and rotor and cleaned the deposits off. Was pretty bad, but the posts still look good.
Did not help. But then I discovered, with engine running, I get a strong vertical spark at the coil wire out from the rubber sleeve to the coil housing. I also seem to hear the sparking inside the distr. cap, but maybe that's normal?

Anyone know what the spark at the coil wire to housing could mean?

Tried to measure the coil and it measures okay, except I can't get a reading between the two posts where the plug is, but that may be because I can't see down there, or have the wrong settings on the Ohm meter.

The "deposits" on the distributor cap terminals are supposed to be there. When the cap & rotor are replaced, silicone dielectric grease is applied to each terminal. When the engine is running, the current jumping from the rotor to the cap crystalizes the grease on the terminals. The grease helps "direct" the current.

The arcing spark is caused by high resistance in the secondary ignition system: Spark plugs, spark plug wires, distributor cap & rotor. 4.3L engines are very fussy when it comes to these parts, use AC Delco only. The only exception is Autolite Professional Series spark plug wires have better specs than AC Delco wires, and they're 1/3 the price.

This image will help when replacing the parts:

http://i561.photobucket.com/albums/ss58/cwhook/96newer43Lplugwires.jpg

Thanks for the tips. I guess I won't really know until I got those spark plugs out.
I had changed them about three years ago to Bosch Platinum 4 but don't read much good about them.

AC Delco 41-993 spark plugs with a gap of .060", distributor cap D328A, rotor D465 and Autolite Professional Series spark plug wires #96871, will give you the best possible performance. All of these parts are good for 100K miles.

Choices are a bit limited where I live, so my wife got cap rotor and wires from NAPA. Going to inquire if they are Delco by any chance, or if I can exchange for what was suggested. Could even only get four plugs at the moment. So far I know it wasn't the plugs, nor the cap or rotor or the main wire from the coil. Coil still arces to ground and after taking the coil out I can see a small hole or pit where the spark came out. Tried to measure the coil with two different multimeters as suggested in the repair manual. The resistance between coil wire and left pin is about 5,700 Ohm, but between the left and right pin I get mixed results 0.9 Ohm with one and 0.7 Ohm with the other meter. Book says it should be 0.1 Ohm between the electrical connectors. So I wonder if I should just get a coil next before changing all the wires. If I use them I can't return them for the ones suggested.

It's rare for a coil to fail as you're describing, but it does happen. Might not be a bad idea to try the coil. As for the plugs, cap, rotor and wires, there are several places online that will ship them to you. Amazon is a good place to start looking, and they usually have the cheapest price.