Same problem here. My FR overheated and I´ve changed the timming belt, flush the coolant, tested the thermostat in hot water and water pump. Found too a "T" connection obstructed inside the radiator. After mounting all, the car works very well, starting the fans in 96ºC and engine temp stays in ~100ºC.
After 2 weeks, traveling on the highway the problem occurs again with same symptons. The coolant fluid is not going in the radiator, but going to heater, IRD, gearbox and oil rad. It appears that the thermostat housing is stuck closed, makes coolant circulating only in the engine block.
I removed all the coolant fluid and pressurize with an air the expansion tank and has no leaks inside the block and hoses.
For tests now I´ll remove the thermostat housing and make a method to fix the valve to stay always open, to the coolant circulate always to radiator.