usually head is warped from heat causing head gasket to blow, take yours off and see if it isnt straight (put something flat accross its length)
if so just get another head, dont get yours machined (its cheaper unless youve got GX), make sure surfaces are very clean when putting new head on, theres one bolt that has to go back in number one position (in diagram)

someone else will give you settings and diagram for tightening cause they posted in a thread not long ago, or you could just search back