It sounds like it isn't getting all the advance it needs at highway speeds. I had a problem with my distributor three years ago. The advance mechanism was binding up in the distributor. I disassembled the distributor, cleaned everything up and relubed where necessary and reinstalled the distributor. It works fine now. The car ran upwards of 230 on the highway when the distributor messed up; it now runs 200 at 70 mph in 80 degree weather.
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