My brothers girlfriens car has this problem of either running cold or too hot.
Example the car can be left to on for a good while and no problems, but after it is driven then stoped and turned off the overflow tank starts to spew boiling coolant and water. The thermostat,relay, hoses, ect have been replaced. im out of ideas any help would be great

 

 

Has the headgasket been changed recently or has the driver ran the car for a long time when hot? If there's a lot of blowby (out of radiator) when your car is running then you may have a blown head gasket, which will do exactly what you're describing.

 

I'd pull the one of the plugs and see if the plug is greenish blue at the tip, its another indication it could be the headgasket. Good luck