turning the heat on causes it to overheat? that is so bizarre, never heard of that before.
Yes, it seems counter-intuitive, but the way the heater circuit is set up, it could happen. See my link in post #4 above.
As long as the heater fan is off, the hot coolant coming into the heater core from the crossover pipe goes out from the core and back to the water pump inlet (and thermostat sensor) at almost the same temperature. This helps keep the t'stat open according to the coolant temperature, and the "balance" mentioned in the "theory" post.
If the heater core is partially blocked, as long as the heater fan is OFF, the hot coolant going back to the pump will still help to regulate the t'stat. But when the heater fan is turned on, the coolant returning to the pump is cooled, and this could cause the t'stat to narrow it's opening, reducing flow through the radiator.
At that point, given that the heater can sometimes be used to cool an overheating engine, we might think that the same would apply with the partially blocked core, but it might not. The partially blocked core will cool the returning coolant, but only the amount that is able to flow through the core, which is, presumably, far less than a properly flowing core.
As noted, it's a "theory" and seems to explain what happens, but it hasn't been reliably tested.