2002 Galant radiator problem???

The other day on the high way I noticed loads of steam coming out of my car so I pulled it over and the coolant overflow resivoir melted. I just replaced it, put more coolant in it and started it up and let it run for a while and it sent the coolant into the resivoir boiling hot again. I have no idea what might be wrong, mabe I need to flush it? Anyone have any ideas? Thanks.

Bad thermost or cracked head. A bad thermostat will cause your engine to run hot and eventually crack you head in which the crack in the head will pressurize and or cause the coolant to boil or bubble over.