My car overheats when standing still and I don't know what the problem is. Can someone

I've been having a problem with my car overheating when it's stalled at a Red Light or something. I've changed/flushed the Coolant, replaced the Thermostat, and I'm now thinking if getting a new radiator. I know it's not the fans or anything electrical because they still come on. Some people tell me it's the radiator cap of the water pump not circulating through the engine properly. If someone could help me out before I make some expensive repairs it'd be greatly appreciated. I have a 2003 Honda Civic EX

When you are driving air is being forced through your radiator to cool off the coolant. So it seems to me that not enough air is flowing through the radiator when you are stopped, here are a few possible causes. The radiator is dirty and not enough air circulation over it from just the fan; carefully clean the exterior of the radiator. After the car heats up stop somewhere, open the hood, and ensure than fan is running while the car is sitting there. (PS if you are running your AC while this happens don't. You AC cooling coil is taking cold air that can be used for cooling the engine fluid. Only so much air passes through both systems so any cold air that is taken by the AC is unavailable for cooling the engine fluid.)

I have heard of impellers on the water pump wearing out so it appears the pump is fine but it is not circulating enough water; this is a slim possibility. Is your belt that drives the water pump tight?

Could be the radiator cap, water under pressure has a higher boiling point that non pressurized water so changing the cap is probably a cheap thing to try before a new radiator.

If you see your car overheating a way to cool it down is to turn on your heater. By doing this you in essence have a second radiator pulling heat from the engine. Not too comfortable to do on a hot day but it may cool down the engine enough to get you to where you want to go.

You need a mechanic! You can't help someone that doesn't want to get the problem diagnosed before guessing at the next thing to throw money at! Have the cooling system chemical test done to see if you have a bad headgasket before doing anything else!

You did not mention the water pump, there's a weep hole on them and if there's wettness or signs of leakage it might mean your pump is going out. What did your coolant look like when you drained it.
Do you know the sign and symptoms of a blown or leaking head gasket, white smoke, sweet smell to exhaust…

If your engine temperature is absolutely fine as you're driving in the highway then your cooling system works as it should (leave your coolant, water pump, radiator, cap, etc. Alone).
The source of your problem is the cooling fan system which means that either the cooling fan itself is not as efficient as it should or it kicks in too late.
First check and replace the cooling fan switch (around 1 inch away from your thermostat) and if still fan is inefficient (or not revolving vividly when engine starts getting hot) then replace it as well.