heating problem again!!!!!

I replaced everything on the car to do with the cooling system. I know I have a minor leak,and when the car loses about 2 cups of water it wants to start to over heat. But if I open up my heater core valve and allow water to flow then the car wont over heat, even when it gets lower on water.. I'm thinking there is a blockage somewhere but I've watched it with the radiator cap off and the water seems to be circulating fine.. I thought it could be my sterio system too, because when the bass hits hard at a idle the motor will stumble a bit, but I turned the sterio off for a few days and noticed it does the same thing... If anyone has any thoughts let me know.... Could it be a weak water pump and not circulating the water fast enough???

Where is it leaking from? It's over heats when low because of an air pocket. If you replaced every thing how could it be leaking?

leaking will cause a loss in pressure, whihc is probably causing your overheating

well the only thing that hasn't been change is the o rings on the water line that runs from the themostat housing to the back of the water pump... that and the head gasket but I'm leaning more to a leaking head gasket for a water leak because there is always a bit oil on top of the tranny. At first I thought it was all coming from the seal in the distributor because I started to notice a bit of oil on the inside of the distributor cap... ... I have good even compression in all 4 cylinders.but probably just a matter of time before it starts to leak into the cylinder