Overheating - mechanics stumped!

I have a 98 sable that is overheating when coming to a stop for a short amount of time. Thermostat has been replaced twice, radiator seems fine,electric fans are working well, no sign that heating core is faulty but the heat is out as well. According to mechanic, everything seems to be functioning properly! Why this OH problem???? HELP!

Lower radiator hose warm/hot? I assume the top is hot. Heater hose IN hot? Heater hose OUT warm? (Should be) Water pump OK? Belt not slipping? Coolant temp sensor OK? (This could indicate overheating even if it is not)

Since heat is not working, sounds like cooling system needs flushing
with a garden hose. There has been several posts about this on the Tauras forum....cars are about same

What is the ratio of coolant/antifreeze in the engine?
A high ratio (90%-100%) will cause overheating.
Ethylene Glycol does not exchange heat. It merely raises the boiling point of water and lowers the freezing point.
If you are running a 40%-60% ratio you might want to do a pressure test on the cooling system and check for a leaking head gasket/cracked head.
Remove all spark plugs and pressure the system up. If pressure drops get a helper to turn the engine over as you observe the spark plug holes for geysers.