Welcome to the Werkz
Like WS said the stock radiators are renowned for the plastic side tanks seperating from the metal assembly. I have just finished replacing mine with one of these units Zionsville Autosport: BMW E39 - E38 Super Duty Cooling Kit
. Not exactly a cheap option, but given that the cost and inconvenience of an "on the road" failure i felt it was worthwhile, but only if you plan to keep the car for any length of time.
Have a look when the engine is cold and you should be able to see the tell tale white residue from the coolant down the seam where the plastic and metal parts join.