Take a look at the pick up coil, sometimes an internally broken wire will cause a problem.....( it might try to make contact till the vac advance rotates it). to test, unplug the pickup coil, & use an ohm meter. try "worming" the wires around to find a break in continuity.....good luck 
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69 longhorn,4" chop,3/5 drop, 1/2 ton suspension/disc brakes,1 1/2" body drop,steel tilt clip, 5.3/Edelbrock rpm intake/600 carb, Hooker streetrod shorties,2 1/2" exhaust/ H pipe/50's Flows , 6 spd Richmond trans,12 bolt/ 3.40 gears....
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