it just sounds like you're low on fluid. running it low for any length of time puts a strain on the heater core. you don't want to have to change the heater core. watch your temperature gauge. don't have one? good idea to put one on - it really puts the mind at ease that you can check the actual temperature instead of just watching the needle go between H and C.
check to make sure the drain **** on the bottom of the radiator is closed. that is one of the biggest
people make after a swap.