Why would a 1996 Chevrolet Impala SS overheat with a new water pump and thermostat?

An air lock could do it. Run the engine to operating temperature then shut it off and let it cool down. Refill the reservior and rerun it to operating temperature. Shut it off to cool. Do not run it to overheating. Do this until you do not need to add anymore coolant. Then run the engine and see if it overheats. Watch the cooling fan and see if it is coming on. If not have it checked out.

My experience with overheating but not actually overheating is i changed radiator, waterpump, radiator cap, thermostat (180), and still had problem with it getting hot after 10-15 mins of driving. I work here in Kuwait where the temp get between 100-130 in the shade, over here we SS caprices, same car, but these vehicles were based on police package due to extreme heat and road condition. Here these clowns only use and can get only 180 degree thermostats, not a 160 nowhere, so what i have found out is that thermostats dont open at the temp it say it is? 180 themostat put in a pot of boiling water 300 degrees actually opens between 224-242 fully open, and closes right 181? weird huh? I had to go old school on it, you can drill holes in top of thermostat this will allow fluid to flow instead of being block and heating up pass desired temp. I guess if i had done this first, it would have saved me some money.