Temp gauge stay on cold.

Possible defective temperature sensor or wiring issue. Check to see if the heat coming out of the vents stays at a constant temperature when this problem is happening. If it goes from hot to warm/cold when the temp guage is messing up, you may have a head gasket isssue developiing

some external sensor has taken a knock etc
sounds like its not getting whats going on outside
or the evaporator has an issue with its drains
perhaps something in the air intakes

The heating coil is probably getting plugged up. You may have heat at idle or slow speeds, but once you get going, there is not enough coolant going through it to stay warm. Get the system flushed or replace the heating coil.

Sounds like the blend door motor isn't working.

Have your thermostat checked, sound like it is not working when cold.

you might have a defective sender unit (temperature) or faulty wiring to the same unit.Identify the sender unit and disconnect it whilst your ignition is on. Temp gauge must fall to cold.If it does, sender unit needs replacement and if it stays high your wiring to the sender unit is shorting with ground

in the case of no heat and slight smell of antifreeze i would back flush the heater core and also check the carpet around the heater vents under the dash if the heater core is leaking the carpet may be wet. find the 2 hoses going to the firewall if you start the motor and let it warm up with the heater on 1 should be hot and one should be cold disconnect them and use a garden hose to flush water in the hose thats cold.

sounds like the thermostat has a problem and needs to be replaced. The thermostat regulates temp and it defective can either stay closed or stay open and I reckon in your case it has stuck open permanently.
Your manual should show you where the thermostat is located.

under the hood of the car on the passenger side you will see a round motor sticking up.thats the blower motor there should be 3 screws holding it in place and one plug to unplug.you lift the old motor out and put the new one in in its place

it may be your thermostat, the gauge on the dash is a dummy gauge, there is only three positions that it sees, dead cold, operating temp, and hot, so even though the needle looks fine it just means it s closer to operating temp then it is to cold,