My car overheats almost to the red line on the thermstat. Then cools down and does fine as long as you are on the highway. Then when you slow down it starts to get warm again. As it starts to over heat when you slow down the heater stops blowing warm air and starts blowing cool air. If you accelerate up to speed after a stop, the heater starts blowing warm air again and the car cools down. Also it always overheats when you slow down like in the parking garage. When you park and the engine is hotter than it should be you can hear a gurgling sound when you shut off the engine. It appears to be pushing coolant back into the overflow tank. Thank you for your help!

The first thing to check are the cooling fans in front of the radiator. Also check for debris that might block the radiator airflow. They should come on when the engine is warm or hot. If they work fine have the entire cooling system checked, thermostat, circulation, water pump, etc.
Since we don’t know how old your car is and how many miles on it, we can’t be very specific.

I suspect the water pump. The key clue is that the heater stops blowing hot when the engine is overheating. This suggests that the water isn’t circulating properly. When you’re going fast, the engine is turning fast enough for the pump to push the water through, but when you slow down, it’s unable to, and so the water in the heater core gets cool as the heater extracts heat from it.

Water pumps can corrode and lose pumping ability:

It sounds like you’ve had repeated overheating episodes. Keep in mind that this can kill an engine in short order, so I hope you can solve this quickly.

I believe shadowfax hit the bullseye: bad water pump.

without make or model and year of vehicle, I think the first step would be to bleed the cooling system. It sounds like you have trapped air in it.

It does sound like a water pump on the fritz. Wouldn’t hurt to ask a mechanic to verify all the air has been bled out first, might get lucky. I’d probably check the thermostat too. Remove it and put it in a pan of water on the stove, make sure it is opening at the right temp. Other things can cause this, but those are the first things to check.