The heater will blow hot and all at once start blowing cold

Hello,

It sounds like you have a blown head gasket which filled the heater core with exhaust gases which makes the heater stop working here are two guides to help us get started.

https://www.2carpros.com/articles/car-heater-not-working

and

https://www.2carpros.com/articles/head-gasket-blown-test

Please run down these guides and report back.

I had the same problem and had to replace the blend door actuator.