Could be that you have some air in the system..I get the same problem with my spider, problem being that the radiator is low slung therefore not being the highest point in the cooling system. To overcome this I jack the car up at the front as high as I can, so the air will collect at the radiator cap and be expelled. Slowly fill the system and give the hoses a gentle squishing, then start the engine and more squishing while it runs up to temperature.
This procedure works for the spider.
Good luck!