I'm assuming that your engine idles normally when the car is level.
Maybe try raising the idle speed and see what happens.
When an engine is on the over-run, the throttle is closed and there is a higher than normal depression in the engine manifold and below the carb butterfly than at normal idle speed. If you have any sort of an air leak, (e.g. worn butterfly spindle, leaking gasket, warped mounting flange, leaking PCV valve or hose) the idle mixture might become too lean to keep the engine running, so it would tend to cut out.
If the engine was flooding on the over-run due to a worn float valve or high float level setting then it probably would be difficult to restart, so I'd rule this out for the moment but it is a possibility due to the high vacuum in the inlet manifold/carb.
I'm assuming that valve clearances are set ok, good compression, good spark, ignition timing correct, HT leads in good condition and secure, especially the coil king lead (coil lead to distributor cap).
Al.