1986 560Sl with a bad miss

Hi..I have a 1986 560SL with 150k miles on it. the car has a severe miss at idle, severe enough that it is almost shaking. When driving, the miss is not noticeable although there isn't the usual power available.

The injectors, distributor cap and rotor are new. last weekend, I changed the spark plugs. I noticed Number 8 was heavily fouled...black. After replacing the plug, I restarted the car, ran it for 3-4 minutes. The new plug in #8 was heavily fouled already.

I replaced that injector. No change. I figure either the cylinder is getting way too much gas (fuel distributor) or the plug at #8 isn't firing.

Any thoughts? this is driving me up a wall.

Thanks.