Diagnosis by notes is tough, but I had a similar sounding problem with my ’53 235. It would start and run ok but would not idle below 1200 RPM. If I tried to idle it lower the engine would slowly loose RPM until at about 500 it would just die.
It seems to be fixed now. The culprit was a vacuum leak at the wiper motor. Mine has a separate speed control for the wiper, it is attached directly to the motor itself with a very short tube and this was leaking.
I believe what was happening was
My timing was set a little late (Power glide engine with an older stick shift flywheel)
The engine running at a high idle was actually running on the centrifugal advance and not the vacuum because of the vacuum leak.
As the idle started to come down the centrifugal advance went to zero and without it or the vacuum advance the timing was just too late to sustain life.
