1995 Ford Taurus GL V6 3.8l. 166,500 miles.

Car is over heating. To around the 3/4 level. Usually it only goes to 1/4 or 1/2. No smoke or leaks. 3 years ago replaced the radiator, hoses, water pump, radiator cap. Havent had the system flushed. Driven only 15k miles since then. I noticed some black sludge in the radiator when I took up the cap off. Also there was a recall on the cooling fan circuit breaker that I didnt have done about 5 years ago. Also this car had a bad heater core that was not replaced. The mechanic at that time did some kind of rerouting to bypass the heater core. My question is could the heater core rerouting have anything to do with this? Or is it more likely a head gasket, or a bad thermostat ( I dont know if it was ever replaced), or the cooling fan. Any help would be greatly appreciated. Thank you.