" Does it help diagnose the problem if it is worse in high humidity"
Yes, it means that you've got a crack in your insulation somewhere which allows the spark/current to bleed off during dampness. You've replaced your ign. cable and dist cap, which are usually the culprits, so those shouldn't be the problem.

See if you can spot any arcing in the engine bay at night.

I came across a tester at Wal-mart that supposedly allows you to check your plug wires while the car is running. Don't know the details but it may help detect arcing. It's palm size and under $10?

I am curious about it starting and running then stalling though. Usually once started, engine heat will eliminate the dampness and it shouldn't stall. So it may be something else also. I think the body of the ignition coil should be properly grounded for spark, so check the holdown bolt.
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1980 245 Canadian B21A with SU carb and M46 trans