Every time i put a load on the engine of my 1981 oldsmobile cultless supreme the heater starts to blow out cold air. I've flushed the heater core and hoses in and out of heater core are hot.
Might be a water pump problem. You could also check you have a good radiator cap on and that air hasn't gotten into the system.
Even failed water pumps still pump, and the engine will still heat up even with no rad cap at all, so I would rule out those. It's more likely to be a cooling system that is not full or air in the system like KenF said that is preventing proper coolant circulation under certain driving conditions. From cold, or at least not super hot, take the cap off and make sure the system is full. Then run the engine with the cap off and see what happens as it warms up. Air trapped in the system will hopefully make it into the rad and escape through the open radiator. You will see the fluid level drop as this happens. Keep slowly filling it up until it stays at a steady level. Once you're there put the cap on and you should be good.
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