1998 BMW 318i cooling system

My BMW is having problems with the cooling system almost about every time I'm in an idle position the car seems to leak, in the past month and a half I had gotten a new radiator, ECT sensor, thermostat, and water pump. I know were the leak is coming from the bottom radiator hose but I don't understand why. When the car is in idle the coolant pressure in the radiator seems to rise, it starts to bubble up until it hits the top then coolant starts to leak, I usually get this problem when I'm in heavy stop and go traffic, my fan works so I know it's not that, at first I thought it could be a bleeding problem because I didn't bleed it right and what I would do while I drove is, stop unscrew the bleed screw and a lot of air would come out with coolant, it still happens but not as often though, also the a/c & heater run fine it's just the fact that I have to keep refilling the coolant because it keeps leaking. What do you think the problem is?