Sounds like head gasket leaking,allowing combustion pressure into the water line,check your spark plug for whitish residue.the head gasket might be leaking in either of the cylinder.your overflow tank is filling up maybe due to excessive pressure from exhaust pressure.
if head gasket leaking, there should be oil residue in d radiator water right?
i'd say, u should cold start ur engine without ur radiator cap..
let all the bubbles out of ur engine & refill water till full..
do that for about 20minutes.. that should solve ur problem..
if problem persist, please see your doctor