I suspect that you have either a bad plug (cracked porcelain?), or that spark is jumping from your plug wires. Have you tried looking under the hood at night with the lights out?--Also do it while revving the motor. I don't recommend driving in 4th gear with the hood open at night with the lights out. LOL

Were the plugs gapped? I use .045 with my MSD6AL. A plug with a broken porcelain can do weird things. I used to be able to tell if there was a broken one on our ignition scope. If you have the old plugs, you might try reinstalling them and see if the problem goes away.

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Dennis
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65' STANG (3330 lbs), 393W NA, Toploader 4 Spd, 4:11 TrueTrac, AFR 205's, Vic Jr. Intake, CI Custom SFT, 750dp, 11.0 CR

11.52@121.29mph