A few weeks ago I was driving my '66 MGB. It stalled as I came to a stop to wait for a train to pass. It started right up and ran fine after that. Today I drove it on an errand. I noticed it running rough. When I pushed in the clutch and coasted, the engine died. I let out the clutch and it ran. Like before, it ran fine for the rest of the drive out and back.
I’ve learned that when there’s no airflow, like in stop-and-go traffic, it will run rough and then stall. I think the ignition coil overheats. Let it cool down, and it runs fine. The engine was completely rebuilt a while ago, and has very few miles on it since then. After sitting for three years, I had it serviced a few months ago. I’m pretty sure there’s nothing mechanically wrong with it. Am I right, suspecting the coil? I have a new one in the boot, and tools in case I need to change it out. Only, it had plenty of airflow today and before I stopped for the train. (Not to do with the temperature in the engine bay, or of the suspected coil, but it runs at a reliable 160ºF coolant temperature.)
Running rough and then stalling in stop-and-go could be a fuel problem - but for me, this is only in hot weather. Ethanol in the fuel can cause fuel to boil in float bowl (SU carbs, right?). Ethanol-free fuel can be found.
Does it a separate ballast resistor for the coil? IIRC, my old girlfriend’s MGB coupe had one and it made it stall when it was going bad. It was going “open” circuit on occasion and that’s when the trouble came.
After sitting for three years, a '66 could have any number of issues. Could be this, could be that… Loose/defective/rotten vacuum hoses might be a good place to start, with no more info to go on.
Not a professional auto mechanic, but I have worked on a lot of old coil-induced ignitions. Over the years, I heard lots of talk about the common problem of heat related coil breakdown…but have only actually experienced it twice. Each time, once the coil became warm, the motor started to misfire regardless of rpm. Whether at idle, or flying down the road @ high revs, it didn’t occur intermittently as you describe, (only happening at idle, then disappearing with increased rpm)… but rather at all times, (once heated up).
By all means, if you have a spare coil laying around, replace it… it’s a five minute job and you have nothing to loose, but my hunch is your problem lies elsewhere.
BTW: My old '70 MGB suffered your same symptom many times, and the fix was never the same (and also never the coil). If I remember correctly, the last time it was water/gunk buildup in the #2 carburetor bowl… but that’s just a bad memory from over three decades ago.