Replaced thermostat and water pump, car is still overheating. What's the issue?

What is your question?

I've checked all hoses (no signs of leaks and all are pressurized and hot where they're supposed to be). I replaced the thermostat and water pump and have done some extensive radiator flushes..making sure to "burp" the system afterward. Heat still works, but not all the time. Also, both fans work and even had fan clutch replaced a few months ago. When i first bought the car i was told that it had some small oil leaks..which after working on the vehicle i noticed a decent amount of residue. Most of the time it takes a while to overheat but it eventually does every time. Could this be the radiator, the heater core, or maybe a gasket leak??

What makes this problem better or worse?

idling and/or lower speeds seems to make it overheat faster

How long has your 2005 Nissan Pathfinder had this problem?

3-4 wks but i live in michigan and it just started to warm up about a month ago. Also bought the car in january (cold af)