What you're describing aftar all is said and done is PHASING. For some stupid reason when the ignition fires, the rotor isn't pointing directly at a pole. This isn't too bad, until the vacuum advance kicks in. Now, as the vac advance increases, the rotor moves farther and farther away from the correct pole until it is closer to the next pole and the spark jumps there.

Look at this Ebay link:
http://www.ebay.com/itm/Mopar-Modified-D...tr#ht_777wt_916 to see what the fix is.

I don't know if your particular distributor uses this kind of reluctor, but you need to move the reluctor with respect to the distributor shaft so the rotor is in a better place when it fires, and the vacuum advance doesn't move the rotor too far away from the correct pole.

Setting the gap is fine, making sure the wires are hooked up correctly is fine, but in the end it's phasing that's my interpretation of your problem.

R.