A bad radiator cap will cause one to do this,it lets too much of the coolant pass by the cap,and runs the over flow jug over on it. Since replace that already check the thermostat also this may be going bad.

When was the last time you change the coolant? You might as well drain your old coolant for possible contaminant. When draining your radiator you can refill with water, start engine, recirculate the water by turning your heather, drain again. Do this for several times till water are all clear. Then refill with 50/50 coolant and distilled water.

On your reservoir DO NOT overfill. There is a coolant level mark on the plastic, if none, leave at least 1/4. Again donot overfill.