[83CADMAN]

I experienced a similar condition with my 83 Eldo. Went though all the same troubleshooting with some results after replacing all the vac lines.
I finally discovered the problem was in the EECS (evaporative emission control system). Try checking the vacuum at the charcoal canister. There is also a filter element located in the bottom of the charcoal canister that may need to be replaced. On mine I found a hole in the side of the canister. I located a replacement at my local U-PULL yard cost me $3 and now no idle stumble or surge. Go figure!