overheating?

hiya, i wonder if you can help me please,i just brought a bmw, 2.5i coupe, lovley car, i fell in love with it strait away, it is the car for me, found out its got a overheating problem, i paid a garrage to supply and fit a new radiator, and thermostat, which they fitted it , they got it up to running pressure and temp, it was ok, and there is no sign of a headgasget gone, no leaks anywhere, or from the exaust, no water either,i had it for a day i done about 10 miles in it, and all was fine, i went to go out on sat night, a day after i got it back from the garrage, traveled about 4 miles and , it was near the red, so i turned back home and by the time i got home it was in the red,honnestly, im realy dissapointed, and am running out of ideas, can you help me please as to what could be the problem, you see it dosnt happen all the time........talk about a brain teaser, a.
yours hopefully,
Steve.