Classic/Antique Car Repair: Overheating when engine turned off, oldsmobile delta 88, radiator flush

Question
1966 Oldsmobile Delta 88, 425ci
After driving a few miles, or 40 miles, when the engine is turned off, the radiator drain rubber hose next to the radiator cap releases between a pint and a quart of coolant in either case.  Coolant system holds correct pressure, tested by Mechanic.  I replaced the radiator cap, no difference in coolant drainage.  Coolant was 50/50 concentration.  1-1/2 months ago engine overheated (cause was leaking heater valve).  I had the thermostat (180° per specs in my CD manual) replaced at that time as well.  The engine does not overheat when I'm driving in stop and go traffic or 65mph.  My Mechanic suspects the radiator is blocking water flow, or the water pump is not circulating the coolant enough.  Could the thermostat have gotten stuck in a near closed position?  I don't have a temperature gauge on this engine yet, so I couldn't monitor the temperature changes.

Answer
The problem you describe is usually only related to the pressure cap
being bad or is of the wrong pressure rating for that engine. Make
sure you have the right cap for that engine. Also a partially clogged
radiator will cause an overflow as well. A good radiator flush should fix that.