Did these cars come with electric fans? If so, you stated you have a new fan, new radiator, good coolant, thermostat, and water pump?
A blown head gasket can cause different symptoms. Yes, one would be the milky oil, but they can also simply consume coolant/water and it will exit the exhaust, And they can also allow exhaust gas/pressure in to the cooling system (sometimes both will happen).
Things to check:
- After you fill the radiator with coolant and burp all the air pockets out, does the coolant level decrease over time, or is it noticeably low at the time of overheat?
- Do you find coolant/water around the radiator cap or overflow cap during overheat?
If level is constantly lowering over time, you could be consuming the coolant. You would likely see a faint white/blue smoke from the exhaust or smell a sweet-ish smell from the exhaust.
If there is water leakage around the caps, you could be pressurizing the cooling system with exhaust gas, causing overflow.
Both of these scenarios are caused by head gasket failure.