Have you tried a lower rated thermostat? Maybe a 180F. Do you have the heater or the water pump bypass hooked to the manifold.
I'm not a cooling expert but I know with my land rover, has 350sbc with no mech fan (no room-like you) if I let it get TOO hot in traffic or in the bush,before the thremo fans cut in, it real hard to cool down. So I have the fans set to cut in relatively early. This way it keeps the heat at bay and never over heats, even on the hottest of days 46degC in traffic. At least not yet!
Is it possible that your engine is getting TOO hot before the thermostat opens and then your cooling system is over efficient causing cold running?
I'm not sure, I'm just chucking it out there... I'll keep scratching my head for ya!