Heater blows cold air when you slow down or stop, heat starts blowing when you accelerate

providing your coolant is full cause low coolant will cause no heat
and your temp gauge is normal in the middle then most likely u have a bad blend door actuator usually on the passenger side

Sound like 1 of a few things. The easiest is air in the cooling system. Bleed it and while car is idling and heat is on full blast. (another posibility is head gasket gone bad)

Do you smell coolant inside of the vehicle. If so then chances are your heater core is going.

Heater coil

glad you found the problem

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These details will help.
Thanks.

jtg

Yeah Ron, the flap that covers the heater core is open and when you're moving the air is pushed over the heater core and into the cab your in. Possibly just a vacuum line of that controls said flap. Check all vacuum lines for good proper connection to these flaps because all of them are vacuum actuated.

I think you might want to start with thermostat replacement and pressure flush the system and as other suggestion check levels when car is COLD!!!!!!

there is a valve on the water pump and one on the thermostat housing open those up one at a time until you get a little bit of antifreeze come out then tighten them back up. All you will be doing is bleeding the air out of the cooling system. my car was doing the same thing after I changed my thermostat.