Overheating Durango

I have a 2000 Durango 5.9 R/T island car that I grabbed for $2500. It's been run pretty hard. It's had some serious overheating issues. It's been flushed, changed the thermostat, new heater hoses and drained and refilled. It'll run fine for a bit then it'll start overheating again. I can usually tell when it's gonna start doing it because the heater will go out and blow cold air into the cabin...in winter....in Alaska. This most recent time, I think might be the thermostat even though it's rather new. Due to the fact that it started to blow hot air out of the heater then went cold then hot again for a second then cold again. I'm really sick of chasing my tail on this and am looking for an actual solve that I can do on my own. Im sick of taking it to a mechanic. Any thoughts as to what the overall issue might be?