Over Heating

My 1999 328I with 120k miles on it is constantly overheating. I have had it pressure checked both hot and cold for leaks and came up empty. Im starting to think its not the lack of coolent that is causing it to over heat but something else which causing the coolent to come overflow once the car has overheated. Any ideas?

could be the thermastat put a new in it should stop overheating