I had a similiar issue awhile ago, actually it turned out to be two things. 1) A crack in the plastic housing on the side of the radiator, took forever to find that one. 2) The radiator cap was old and wasn't sealing properly anymore. There could be other problems, but if your only symptom is slow coolant lose, start small. If you have other symptoms as well, post them up and maybe someone else chime in.