I am not getting much heat out of my heater?

I recently had the thermostat replaced and a new water pump put in. The mechanic told me that there might be an "air bubble" blocking the flow of coolant to the heater core. I am also finding that coolant is always filling up the overflow bottle and spits out the hose that exits it. Only once in a while will I get very good heat. The heater core is in good shape and not leaking. I have had to add coolant a few times since the new water pump. I attribute this to the overflow bottle sucking out the coolant and spitting it out. There are no leaks anywhere with the cooling system either.