Overheating please help

Hi I currently own a bmw 520i se [e39] 1997. Vin BH 31436. I have owned this car for about a year now and had no problems until now. I had just driven about 20 miles when the temp went to red all of a sudden. When I pulled over found the rad had blown. Got a mechanic to take it away and had a new rad fitted. When I picked up car I drove it home about 15 miles no probs[or I thought] untill I got home and heard strange noises from engine. When I looked, saw water bubbling and coming out from cap on top of expansion bottle. Thought maybe cap had not been put on correctly. So refilled water and bled system only to find I cant drive the car more than 2-3 miles without it overheating and spewing water from the cap. I have checked all hoses, changed thermostat. When the car is just ticking over it doesnt seem to overheat. Im finding that when I take off the two bleed screws, the water would rather seep out from cap than come out of bleed screw holes. There are no colour changes in water or oil. So im at my wits end. Please please help thanks. Andy