My guess is its the water pump. Depending on the setup, you may have installed an incorrect water pump. Try replacing the water pump with a new one. If you are running a serpentine setup use a reverse rotation. Heck it might not have even been your fault. I have gotten incorrect water pumps brand new from fairly big name stores(and catalogs). There were some years of trucks where they came with both serpentine and v belt setups. The part numbers might be close enough to make the mistake.