Hi Veek,
All good sugestions, IMHO it's the alternator charging capacity at idle. Check your volt gauge when the tachs' bouncing and you'll see 10>11 volts. Anything under 13.4 starts upsetting electronics including ignition boxes and EFI computers. What we've been doing is fitting the smallest v-belt pulley possible on the alternator. Most cars are running aluminum pulley sets, the crank pulley is smaller than OEM Ford and the Alternator pulley is bigger. This just slows down the alternator so much it cannot keep up at idle. Increase the idle RPM or the speed of the alternator via pulley dia. and you problem will go away. That should do the trick.
Cheers
PS Pic is of latest install 347 Ford Racing with Webers, short accessory drives, low and forward engine mounting. The distributor is under the bar below the bulkhead window! No problem seeing out the back either...