My car is occasionally overheating and heat turns from hot to cold what could cause this?

Check the antifreeze level, it may be low if it is then have the car checked for leaks. you may have a bad intake/head gasket and leaking internally.

Here is a guide to check for a head gasket failure which is common.

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

Please let us know what you find. We are interested to see what it is.

Cheers

A head gasket problem fills the heater core with exhaust which doesn't heat up, these two problems are related.

Not necessarily, other htan a coolant use and heating problems which you are experiencing.

You might have a stuck thermostat.

If air is trapped in system, the coolant level in the pressure container would drop when the engine is cold.

This is a description of what would happen.

COOLING-AERATION

Low coolant level in a cross flow radiator will equalize in both tanks with engine off. With engine at running and at operating temperature, the high pressure inlet tank runs full and the low pressure outlet tank drops, resulting in cooling system aeration.
Aeration will draw air into the water pump resulting in the following:

High reading shown on the temperature gauge.
Loss of coolant flow through the heater core.
Corrosion in the cooling system.
Water pump seal may run dry, increasing the risk of premature seal failure.
Combustion gas leaks into the coolant can also cause the above problems.