you need to bleed your system.
when you overheat and a lot of the coolant boils off (it expands and overflows out of the pressure cap) and is replaced with air. When you top off, the reservoir is full, but there is still air in the engine\top of the radiator.
Open the bleeder valve and let it bleed out, it may take a while. Pelican parts article describes how to do this.
There is also a reason that this happened in the first place.
1. Hose wasn't clamped tight enough (whoever may have worked on it could have tried to avoid crushing the radiator hose neck since they are fragile and force you to replace the entire radiator) -tighten but BE CAREFUL because you can destroy your radiator by turning that screw too much!
2. Thermostat stuck closed and pressure built up, blowing the hose open. - replace
3. Your pressure release cap (the radiator fill cap) is bad and is holding more pressure than it is suppose to - replace
4. Your car got mad and you are now in for one hell of a coolant part replacing binge.
Just remember - bleed it out thoroughly, air in the system is bad. The reservoir on the side is just that, a reservoir. If there's air in the top of the radiator, but the fluid is below what there is on the reservoir, it WILL NOT fill the engine. You must bleed it out through that hole.
Check this thread:
http://forums.bimmerforums.com/forum...d.php?t=470644 and read my last post. The antifreeze choice is yours, but if you just want to be safe, the BMW coolant isn't to expensive.