1995 Ford Mustang 3.8

Problem:
Car runs hot for about 15 minutes before cooling system will work. During this 15 minutes of running above normal temperature, the upper radiator hose is hot, feels like a small stream of coolant is passing, but less than I would expect to be normal, sometimes the coolant boils over in the coolant reserve, sometimes upper radiator hose concaves on itself from suction, fan does not come on. After 15 minutes of running hot, boom! seems the thermostat will open completely, the temp gauge will quickly go down and the car will cool properly including fan coming on, so long as the coolant has not boiled out of the reserve to much.

I have fixed or replaced the following:
radiator(new, OEM)
water pump(new, Autozone)
thermostat (twice)
I swapped out temperature sensor for the PCM with a used one
there are no external leaks
there is no coolant in the oil
temp gauge is working properly
recently fixed DTC code for o2 sensor
there is voltage to the fan and works properly after initial overheating

My most recent attempt to fix the problem, I replaced the o2 sensor, but I was convinced the thermostat was in backwards, so I took the thermostat out. It was in properly, with spring towards manifold, but the vent was at about the 2:30, 3 o'clock position, rather than the 11 or 1 o'clock position it should have been in.

I am stumped. I don't know what to try next. Any suggestions?

 

Not burped well? That sucking in part sounds like it's low on fluids.