blows cold out of the vents sometimes when the heater is on?

I just had a blower motor resistor with auto temp control replaced

This can happen when the coolant level in the radiator is low. Here is a video to help you check the coolant and be sure the engine is cold before you check it.

https://youtu.be/GSE9skKnOgY

This guide can help as well:

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

Please go over these guides and report back.

Heat does come out, then all of a sudden, the heat coming out of the vents turns cool while heat is still coming out of defrost. Then it will go back to hot air. There should not be any air coming out of vents because it's on defrost.

If the coolant is full, then it sounds like you have a mode control actuator that is out. here is the location so you can check it out and replace it to fix the problem. Check out the images (below). Please let us know what happens.