Engine Skip

I just rebuilt a '62 aluminum 225. The engine cranks good and idles smooth. When it builds up a little RPM's, the engine starts to skip, but when accelerating, the skip disappears and the engine pulls strong. When you level out at cruise, it starts to skip again.

I installed a mild cam (210/212 duration), Holly 390, headers, MSD Blaster2 coil, & Mopar EI. I have set the valve gap at .018 on both (per the cam manufacturer), checked for vacuum leaks, flushed the fuel tank & lines, set the timing, & checked the plugs & wires. I am baffled at this point. Do I need to use different plugs since it was converted to EI? Also, I was told by a mechanic that I might need to change the vacuum advance springs in the distributor? Does anyone have any suggestions?