1) Make sure the thermostat is opening at the correct temperature.
2) I assume you have filled appropriate coolant amount and bleed air out of the system.
3) Make sure your radiator fans are coming on at the right time/temp. Do you feel hot air being pulled through the
radiator when the car is up to temp. and the fans are "ON"?
4) make sure you have good coolant flow in the radiator, sometimes water pump impellers wear out
due to neglect of the coolant and it no longer adequately pumps the water, or thermostat is stuck shut.
5) verify you have heat exchange from the water to the air, when car is up to temp., and the fans are "ON",
the top hose will feel hot (should be about 195-210 degrees depending on thermostat), and the bottom hose
going into the water pump should feel cooler (to the touch). Get something to measure the temp of the hoses
(top and bottom) and compare the difference, should be around 30 -50 degrees difference, the larger the difference,
the better. If your radiator is working properly, you will be able to feel the difference in temp just by touching them.
Best thing to do is get a little hand held thermal infrared temp gun, they are not expensive, and are great tools
for diagnosing all kinds of problems.
Hope this helps!