Hi Steve,
From my experience, this condition is the result of a number of issues including a miss-adjusted carburetor set on the rich side in combination with heat build-up in the engine compartment. Even though the heat will drop when the bonnet is open, by that time the build-up of carbon and un-burnt fuel builds in the combustion chambers to cause the reduction of engine RPMs, it will continue. At driving speed, yes engine bay temperature will drop but an extended increase in RPMs will usually burn off the fuel build-up and blow out the carbon to overcomes this RPM-bogging condition until at idle again.
I don't believe that vapor lock is the issue unless the car builds temperature to cause substantial overheating at idle and the car is difficult to restarted (air-block develops in the fuel line). However, making sure the engine compartment is maintained at a cooler state through the creation of a more efficient directed air stream created by the addition of even the simplest of a fan shrouds placed around an aggressive fan and more efficient radiator core.
Again, we are addressing the build-up of an operational environment that will produce increasing amounts of carbon and unburned fuel that will eventually overwhelm ignition. Eliminating or slowing the development of this operational environment will require a hotter and more precise ignition along with the more efficient management of engine operating temperature through the installation of fan, shroud, and improving radiator heat-exchange efficiency. Any one of these may be already present in your car and has probably improved an even worst condition, and any one of the above may help and improve your present situation, however, I believe all are needed to increase the margin to make this condition disappear or become a rare condition.
Good luck,
Ray (64BJ8P1)