Why is my 6-cylinder 1991 Toyota Camry with a V6 engine overheating?

Is the oil changing colour to a milky white? If so, that indicates a blown head gasket. Let the engine idle and remove the radiator cap. If You see bubbles, that means air (exhaust gas) is getting in, and as the cooling system is supposed to be sealed, a blown gasket is usually the cause. Your last descriptive sentence also indicates that your cooling system is being over pressurised, and thus, causing coolant to over flow in to the catch can.