'93 Toyota truck overheating when cabin heater is on?

[cfoster2030]

I have '93 Toyota truck with a 4 cylinder 22re engine and when the engine is cold and I turn the hot/cold lever over to heat the temp Gage goes all the way over to red zone. When I turn it back to cold the temp drops back to normal. If the engine is already warm it usually doesn't have this problem. If I don't turn the heat on it stay well on the normal side below half or more usually. My sister had a '94 Toyota and it did the same thing as well. Her truck was almost brand new way back when and it still did it. It might be a design flaw. I have replaced everything associated with the cooling system as well. Replaced the fan clutch,pump,belt,thermostat, radiator, and flush the cooling system, heater core out several times and top off the cooling system while running to make sure there aren't any air bubbles. If anyone has anysuggestionss or fixes I would greatly appreciate it. Thanks in advance.