I'd perhaps try flushing the radiator, you could do this in the car by disconnecting both radiator hoses and putting a garden hose or similar in the top and running water through see what comes out. If you don't get much this way remove the radiator and try putting the hose in the bottom, shake it about as it might be blocked and filled with crud. If you can get it to flow easily and clear that's good if not you may need a new radiator. If it flows easily from the start and there's not much crud it the problem may be elsewhere.
Fill with water until you get it running ok as it will save buying coolant all the time unless it's going to freeze overnight or something.
