Also make sure that the waterpump is OK, as they get older and especially if they have been run without proper anti-corrosive coolant, the blades on the impeller can corrode. Also, the surface on the timing cover that the impeller runs against should also be smooth, otherwise it will cavitate and/or not flow well.

perhaps the 'clutch' on the fan isn't grip enough? i've seen 1 or 2 cases like these, the fan is too slow when idle. replace the silicone fluid inside the clutch, problem solved.