I swear I answer this 5 times a day. I'm on my way to master contributor just off this question alone!
This is the situation:
The automobile is full of coolant and the reservoir if full. After running the car for a few minutes it overheats then cools once shut down only to do it again and again. Boiling over the reservoir if opened. The coolant is replaced and it happens again. I'm frustrated! Don't worry this is a very common problem with an easy fix.
This is the problem:
The engine coolant was at sometime in the past low enough to let an air pocket in behind the thermostat. Usually engine work was done that allowed the coolant level to drop and this problem resulted. All thermostats have a tiny hole or notch cut in them to avoid this but because of degrading gaskets, loose silicone blue goo, and contaminates these vents become blocked. This traps the air pocket behind the thermostat. Your car now needs “burped”. I'm not trying to be funny, that's the technical term for it.
This is what is happening:
The air bubble behind the thermostat keeps hot coolant from coming in contact with the thermostat. Without coolant touching the thermostat it will not open. If it did the system would burp itself and the problem would end. So, the thermostat stays closed and the coolant boils over entering the radiator backwards, cools and the cycle starts again. If you were to open the radiator cap it would shoot coolant out like a shotgun then sit and percolate like a coffee pot.
How to fix it:
You can burp your car by finding the highest point in the coolant system behind the thermostat. On American V8 cars this is almost always the heater hose. As for imports you'll have to use your best judgment but it will probably be the heater hose or carburetor heater. Some fuel injected cars use manifold heat too. Anyway, find the highest point and vent it by removing one end, the highest one. Now, if done correctly the coolant system will now accept more coolant, usually at least another gallon. That is how you'll know if it was the problem, by all the extra coolant you can now feed the system. Keep adding until if flows out your vent. Replace the vent and top off the coolant again. Your problem should be gone.