Basil-leaf

This is a very weird scenario and I dont know what happened, neither does anyone I ask.
Recently I have been working on my heating system since its freezing here in winter and not having heat sucks. So I flushed out the entire coolant, unclogged the heater core and put a new thermostat in.
The heat worked miraculously.

A few days later I was heading down the highway and all of a sudden my heat turned to frigid cold...
A few more miles down the road the truck started overheating and the overflow can from my rad started spraying coolant and fumes. I pulled over to let it cool down and took off my grill cover (made to keep the engine warm in winter) off as well.
Kept driving and noticed it overheating again and still constantly blowing out cold air. I also noticed that the idle was very low (about 500rpm) and the oil gauge was very low as well. It did this about three times on my way to my destination. Once I finally got home I topped off the coolant and it hasnt overheated since then but I am driving in the city. All the other symptoms are still active though.

These issues are very odd and I have NO idea what the problem could be... Any help at all would be greatly appreciated...the cold ''heater'' is deadly in already -40 degree weather.