[jammyjim]

sounds like you are getting too much fuel. Black smoke indicates a rich fuel mixture. I had the same problem with my old gl. It turned out to be a loose connection from my O2 sensor. I would start with your sensor connections. Especially your O2 and water temp sensors (on an 8v, it should be mounted on the block inline with the inlet hose from the radiator). When you start your car, the fuel mixture is rich and as it heats up, it leans out. If your ECU is getting a bad read from your O2 or water temp sensor, It won't lean out and your plugs will start to foul. Then, you start to get studdering, powerloss, and black smoke.

Good luck