[rbohm]

you do need an overflow tank, but it sounds like to me that it is just that you have a bit too much coolant in the system. try not filling it after it does this and see what happens after the next drive. the cooling system will seek it own proper level and will puke out excess coolant. it has to have the expansion room. that said, if it does it again with you not refilling the rad, the you might have the begining of a head gasket problem or other cooloing system problem.