Radiator boils over into overflow tank even while cold

If the engine is cold then the radiator coolant can't be "boiling" because it requires heat to boil water . So I am going to guess that either you have a blown head gasket that is pressurizing the coolant system past the point that your radiator cap is designed to hold or your radiator cap is bad and allowing the coolant to expand over into the overflow tank before it should.

Sounds like 644 is on track , try a radiator cap first , its cheaper

Change thermostat,check hoses by squeesing them while car is on.what is car make also .some car need to be bled or take out air out of radiator lines,hoses,is nothing major but it has to be check out ,like i am telling you

This is caused by either a leaking Head Gasket or a warped or cracked Cylinder Head.

The only thing that could cause this is a blown head gasket. take the car in to have it checked with a test they call a "block check". what it does is test for exhaust emissions in the cooling system. average cost is 50 bucks