Make sure all your hoses are good. Sometimes when they are older and get bad, the will blister on the inside and restrict water flow. Also make sure the water is flowing thru the entire engine and making its way out the exhaust, between debris sucked up and rust flakes from the cast iron, a piece or pieces may be causing a restriction in a port. I have even seen engine blocks filled with debris so bad you have to disassemble the motor, remove all freeze plugs, and clean out. Rare, but still a possibility. Start with the easier stuff and work your way, which it sounds like you have. Check to see if the water is flowing thru the exhaust manifolds to the risers. I have found the passages between the two stuffed with debris also, which is where the hot water needs to exit to allow fresh cool water in.
Good luck.