1995 3.8L Mustang unusual overheating issue

I have a 95 v6 mustang and about a year ago I completely rebuilt the engine. Everything on the motor was replaced with new parts (water pump, thermostat, ect.) But I did not replace the radiator or heater core since this was my first motor overhaul and I was a bit naive. I flushed out the core and radiator and water came out clean so that was why I didnt replace them. Long story short a few months ago I noticed a lot of rust buildup in the radiator so I decided to replace it.

The car had no overheating issues at all, I just wanted to replace the radiator for a new one. Now my car overheats to the point where steam comes out of the radiator cap. I have a 180 degree thermostat in it that I have replaced numerous times. I have bypassed the heater core. I have a brand new radiator cap. I even replaced the CCRM (constant control relay module) because of other reasons but also since I found out it controls the fan I wanted to replace it.

Here's what is most unusual about my issue, I can drive on the highway and the engine will stay nice and cold but once I get off the highway and driving around typical city conditions, it will start to overheat. Also if I have the AC ON it seems to reduce the overheating issue.

This is the most backwards thing I have ever dealt with, with any car and any help whatsoever will be greatly appreciated.

A few side notes: no it is not the head gasket (pressure tested), not a thermostat (after 5 replacements in the past 2 months, I don't think so), the fan does work (no grinding or electrical issue)