Diagnosing car overheating issue?

My cruiser has been doing a soft rumble/vibrate every few seconds and then overheating. The first time it happened when I opened the hood there was NO coolant in the reservoir so I filled that and it was fine for a little while, I checked the coolant again before I drove it again and it was almost empty again so I thought I had a leak and filled it back up, but the coolant level stayed the same after that and it was fine driving for about 24 hours so I thought I must've just needed the coolant badly and sucked it all up.

Then it started overheating again so I stopped and checked the coolant level and it was fine, so my friend said maybe there were air bubbles in the cooling system and I needed to bleed the radiator to get the air out, so I did that, and there was a ton of air that came out ; I drove home about 30 minutes, then again to work this morning with no issues so I thought I fixed it.

On my way home I was sitting at a light for a while and it started vibrating/rumbling and the heat level was rising so I started blasting the heat in the car on high to hopefully get myself somewhere I could safely stop before it overheated, but once I started driving at a decent speed it cooled back down to normal operating temperature. Then I turned the heat off and it was fine for a while, until the same thing happened again.

So, I've noticed the trend that it starts overheating when I have been sitting still/idle for a long time and/or been going at a low speed in stop and go traffic. But when I turn the heat on high and start going at a decent speed (25+ or so) it cools back down.

Can you help me diagnose the issue? I think it may be the radiator fans or thermostat from what I've googled but I don't know how to check them.