I just put a new engine in with a new water pump but kept the old radiator. I was having massive overheating problems before but it was because I had a chunk missing out of my heads. I got the car started today but when I drove it the engine temp seemd to keep moving around from the middle to the top of the guage. It didnt move very steadily either. When i put the clutch in it usually came back down to about half way pretty quickly but when i gave it gass sometimes it would go up and other times it would stay in the middle. What could be the problem??