[Dead Parrot]

Is there oil in the water and/or water in the oil? Either is a common symptom of cracked head/block or bad head gasket.
Does it run smooth until it overheats?
Is there a bunch of blue smoke(oil) or white smoke(coolant)?
How do the plugs look?
Do you get a lot of heat out of the heater?

If there is no fluid in the wrong place, no smoke, smooth running until it overheats and the plugs look normal - guessing bad/plugged radiator or a block with really fouled water passages.

If you keep overheating it, you risk turning a possibly good motor into a definitely bad motor.