Intermittent Misfire

First off, I SUCK at diagnostics. I've known since I was a kid that "parts-replacement" is the worst way to solve a mechanical problem, and this situation reinforces that. Even though I know better, I've been replacing parts in an effort to solve an intermittent misfire at idle/low RPM of at least cylinder one on my 1967 Dart with a 273 (stock aside from a 500 cfm Edelbrock AVS2 on an SP-2P intake through cast iron manifolds and duals).

Here's the issue: At idle with a timing light, I can clearly see the timing light skip a flash every so many rotations of the engine. I've verified this with two different timing lights. Once the car is warmed up and the choke is off, there's also a very faint stumble when you roll into the gas off idle or light cruise.

When this began, I checked the distributor cap and found two opposing spark plug wire ports (and the corresponding plug wires) blue-green with corrosion. This was cylinders 1 and 6. The other ports looked shiny-new (the cap wasn't old). The two corresponding spark plugs also looked black and sooty, while the others looks good.

Since then, I've replaced (in order):

• Distributor cap, plug wires, and spark plugs – no change
• Timing chain (was stretched) – but no change
• Rebuilt distributor with new rotor (provided by Halifax Hops, converted to Pertronix, with a spectacular ignition curve) – car runs WAY better, but no change in the misfire
• Coil (MSD Blaster 2) and another set of plugs – no change

I figure if the misfire was compression-related, it would be more constant/consistent, so I haven't pulled the valve cover or performed a compression check yet. Even with the misfire, the car runs SO well. I just imagine how much better it will be if I can solve this. What obvious possibility am I missing?