It was happening to me.
Car would heat fine when moving but cold air started when at stop lights or parked.
I also noticed slight overheating as well.

Turns out my overflow hose had a tear in it.
My antifreeze was low because of it.

I replaced the hose. Filled up the radiator and the heat works perfect again.

I would check for bad hoses.
It could also be a bad water pump.