Heater is blowing intermittent cold air if not high idiling... Help!!

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My cars heater works great if I'm driving but if I'm at a light or idiling for a short period the heater starts to blow cold air. It's my only transportation with my kids and I can't afford a lot. I have been looking online and everyone said it was low coolant so I found the right one (red) and put some in but it doesn't fix it. How do I figure if it's the right amount of coolant and/or is it something else I need to do? I saw thermostat but a lot of the posts said it would be overheating which it's not, and it still blows hot air in high idiling. Please help me get ideas to fix this... I can barely afford my bills let alone a mechanic. Thank you for your help!