When car slows down heat cools down

Re-flush the system again, I know but it's best you do. Buy a lower degree thermostat and see how it does.

does the cooling cut on and off? if is running all the time then that may cause loss of heat, is there any coolant loss?

the pump will pump water at full blast without the thermostat
unfortunately if the radiator cores are blocked then that description is indicated
try replacing the radiator with a new one and refitting a thermostat
with a new radiator you are very likely to get an over heating problem as the pump will pass water quicker that it can be cooled and so the water temp will continue to climb

Defective thermostat probaly staying open, Or a blockage in the cooling system , Flush out the whole ststem, End don,t have the front of the vehicle raise up,

Maybe your radiator is clogged.

Has anyone tested the new temp unit to make sure it's functioning properly? Or is their possibly a mechanical thermostat in it that's hung and needs to be replaced?

How is the interior heat? Is it normal or are you only getting about 1/4 of what it used to be like when it was newer? Are you running a summer thermostat or a winter thermostate? The thermostat may be defective and not closing all the way and needs to be replaced or the Coolant Temperature Sensor may be Defective and needs to be replaced. I love these cars. Good Luck and have a good XMASS.

Ihave a 2000 Dodge Durango i have put a new radiator and the gaskets came back good um i replaced the coolen fan also the thermostate and tested it it it came back good when i run it its fine but when i am in line its hot when i drive it cools down........

not at all times can thermostat contol your cooling system, check for water leaks, water pump, radiator fans(check for speed) if all your cooling systems are good , replace your cylinder head gasket,

It actually sounds to me like you're low on coolant. My Jeep leaks coolant and whenever it gets low the temp gauge will go real high and then drop, and the heater doesn't work well. This is because the cooling system of your car is supposed to be a closed system, full of coolant and no air. When coolant leaks out, the space it used to occupy is now occupied by air, which does not transfer heat well. When 'air' is passing through your cooling system, no heat can be transferred from your engine to the heater and radiator, resulting in a hot engine and no heat at the heater. Then when a pocket of water passes through the system, the temperature gauge quickly falls as the water absorbs the heat from the engine. The hot water that cools the engine is where the heater gets it's heat from as well, so when water passes through the heater core, the heater works, but when it's filled with air, it doesn't.

Hot water runs through the heater core regardless of whether or not your thermostat is open or closed. That's why your heater works in the winter even before the engine is at normal operating temperature. The fact that the heater stops working is a good sign that the thermostat is probably not the culprit.

As for the water pump. If the water pump was bad, your temp gauge would go into the red and stay there. Water pumps generally don't work intermittently. Either it's good and ot works, or it's bad and it doesn't.

However, if you are in fact low on water, as I suspect, it means you probably have a leak somewhere. The leak could be in the water pump housing gasket, so depending on where you take it for repairs, they may try to sell you a new water pump anyhow. So just beware of that.