90 4-runner Coolant Mess!!

I posted a problem before and had replies that I might have a bad head gasket. After taking the 4-Runner to a Toyota mechanic he checked and fortunately the head gasket was OK. The mechanic found burnt spark plug wires and a bad distributor that were replaced. Also there was a loose radiator cap causing a coolant leak. When we bought the vehicle home I noticed the heater was blowing cold air. Thinking there was big air bubble I refilled the radiator with distilled water, turned the heater all the way with the heater motor blowing to get rid of the air bubble. I went into the house for 15 minutes came back out the radiator was steaming, and had blown and spewed coolant all over the engine and driveway. I did notice a coolant drip coming from underneath the engine toward the back of the engine. I quickly turned the engine off and when the motor stopped I heard a klunk coming from the right side of the radiator. I then let the engine cool down, refilled the radiator, kept the heater control all the way up to hot along with the heater motor on, then turned the 4-runner on again. After the temperature reached the normal level on the temperature gauge, it started to steam, boil, and spew coolant all over the place again. What is going on a plugged radiator or bad heater core??:banghead:

It could be your thermostat. But it does sound like a head gasket too.