Radiators that are cold at the top usually means trapped air. Air expands a lot more than water, and in an open vented system, the expanding air will push water in to the header tank. This may well be the reason you are getting water coming out of the overflow pipe.
Even although my central heating is a sealed system, I needed to bleed it quite a bit when it was commissioned. Even now, I still need to crack open a bleed valve now & then on the radiators.