Ahh... gotta love overheats. It could be -anything-!
For me, it happened on 100F and hotter days with the AC on. Here's what I did:
Had radiator removed to have it "cored" -- essentially removing the tanks and then gutting it out. My mechanic took it apart and noticed that the radiator was already in perfect shape, so he just put it back.
He replaced the water pump, which he said was making a whining sound.
The next day I washed and cleaned my engine compartment. I drove it around, then later that day opened the hood to show off my motor to a friend. Saw some water spots around where the radiator cap was. Replaced the leaky radiator cap with one that had a locking lever on it, and those water spots never came back.
So it was the water pump and radiator cap together plotting against me!
My friend, these problems cant really be diagnosed.

Start with the radiator cap, then the thermostat, then the water pump, then find a place to remove and gut out the radiator (sounds like you already did that). Hopefully some or any of these will solve your dilemma. Best of luck to you...
-Jesda