What part did you replace it with? The dealership part or the Dorman kit that has everything all in one - housing, thermostat, etc. - or the $18 cheap aftermarket housing only? Did you use a new gasket or the existing one? What did you torque the bolts to?
Take the housing off and use your fingers and feel around on the head. Is it smooth or bumpy? If bumpy make sure all gasket or other material is cleaned off. If all gasket material is cleaned off and it's pitted, then you have a problem.
One possible solution to a pitted cylinder head is to dispense completely with the housing to head gasket. Instead you take the housing and fill the gasket channel up with epoxy as well as the other flat areas on the housing. Then sand, file or mill the housing area smooth and use a regular flat gasket, and a -lot- of gasket sealer. Somewhere there's a writeup by someone who did this and he said it works well. You can use your old housing for this.