2003 Chevy Impala overheating again!

I am in desperate need of help, my chevy imapala 3.4v6 keeps over heating! About two months ago I replaced my water pump and that seemed to work for a bit. It started over heating again so I took it to a mechanic and he replaced the fan relay (the fan that cools the radiator) and the tempature sensor. That seemed to work for about two weeks and now it is running hot again and now leaking collant. My mechanic then told me I would have to replace the radiator. So a few days ago I replaced my own radiator (I removed and drained the old one)..filled it up with fresh coolant..seemed fine for a couple days..and today here we ae again! Everytime I stop at a red light or get stuck in very slow traffic it starts over heating very bad! (I mean red line, "Flashing HOT coolant temp") I pulled over and turned the car off and lost power. I had to get jumped and hardly made my car to my driveway without blowing my engine. Could he fan be bad again? Or could there be air pockets in the radiator? The coolant is bubbling very bad! Is anyone else having this problem or have any clue as to what is going on? This is my only car, and honestly up untill recently it has been great! Please help if you know of anything I should be doing or any GMC recalls that this could have to do with. Thank you in advance. Hope to hear from someone soon.

Did you replace the thermostat?

Sounds like you have a stuck thermostat and its stuck closed.
Check that or you have a blockage in the engine.....have it flushed.
A stuck thermostat will overheat the engine and the fan sensors will not work.
You are not getting coolant though the radiator.

To test to see if the thermostat is stuck, grab the radiator hoses, after the engine temperature starts to get hot, one hose will be hot or warm and the other cold. Let me know how you do.

John