Troubleshooting stumble at idle in gear

It "feels" like an ignition problem, but I'm not sure how to troubleshoot it. I may be missing something easy.

Here's the problem. After installing my new (reman) Super Six BBD carburetor, I have an intermittent stumble at idle, only when in gear (never in Park or Neutral). Some days it's there, some days it isn't there.

When it's there it happens at irregular intervals averaging about once per second. It often causes the engine to stall completely (embarrassing when you are showing your ID at the gate and the guard has to watch you turn the key again). With the old carb, instead of a stumble I had more of a hiccup, just a "putt" out the tailpipe with no stalling. Idle mixture was about the same, around 13 AFR on the dash wideband gauge.

Vacuum is 18.5" in Park, drops to 16 in gear and drops further when this stumble is happening. Ignition is stock Chrysler electronic ignition. Plugs, wires, ballast resistor and coil are all pretty new. I have raised the idle to 900 rpm in Park, but that doesn't really help with the stumble. It drops about 200 rpm in gear.