i have a 87 trans am with a dual style coil, 6al msd box, have one of their drop in wiring harness. having real erratic tach needle movement around 3500 it would swing from 3000 up to 4000 and back down,the engine and tranny you could hear it still speeding up, then it would settle down around 4,000 and then continue to keep climbing only under wot. if i put the wiring back to factory, it doesnt do it.called msd and they told me that there is a resistor in the wire like 18 to 24 inches from the clip, which is the big white wire. found a clip just like the power clip that powers your ecm, right behind the distributor, i figured that was it, so i unclipped it and put in a jumper wire. went out and tested it, now the needle starts swinging around 4000 now and only swings a little bit like 4000 up to like 4300, and then it stops. talked to 92zcamaroperson and he has the same setup and he doesnt have any problems,