Just some food for thought,
I think all of the suggestions are good.
Since you just replaced everything, The 1st thing I'd check is to make sure the thermostat is either working or facing the correct diretion.
To check if it is working put it in a pan of water on the stove. boil the water. As it get's hot it will open. If you need to know what temp it is opening use a candy thermometer. (or a digital guage if you have one)
If you don't want to pull it appart to check it. Start w/ as cold motor. Pull the cap off the radiator, fire it up and run it until every thing warms up. You should see water circulating through the radiator. It will push some water out as things warm up so keep your pets away (antifreeze) (If your neighbor is a tree hugger make sure you do this on thier front lawn
)
By doing this you are checking that your water pump is working and the thermostat is also working.
If it's over heating on the freeway and not around town, you may also want to check out the thermostatic clutch on you fan. If it does not lock up you will not pull enough air though your radiator, just because it's new doesent guarantee it is working right
Also, your heater core is part of your cooling system (easy place to trap air). so while checking for air in the cooling system, dont forget to turn the controll to heat while you are running the motor / filling / burping the system.