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It is more than likely your water pump. Can you run it in park with the radiator cap off. The cap on the radiator not the overflow tank. When the temp opens the thermostat you should see flow if the pump is good no flow it is probably bad. If you don't see flow check the upper hose for temp and the lower hose. If the upper line is barely warm and the lower hose is significantly hotter, it is either water pump or thermostat, but since you replaced the thermostat I would be back to water pump. BTW you should use either a contact thermometer or an infrared to check hoses, although it was done by hand in the old days.
Usually, hoses pop off if the radiator/expansion tank cap is incorrect/broken. Find out if this "mechanic" changed the cap, if they did, get the old one back or get a replacement from the dealer. I have never gotten a dependable replacement from any parts stores. If they didn't change it, change it anyway and see what happens.
You didn't state when it overheats, if it is while driving the fault can be : belt driving water pump slipping , lower radiator hose collapsing, most common is clogged radiator. Start the engine with the radiator cap off when it is COLD and observe the flow in side. If you see really fast flow of coolant you have the bottom portion of the radiator stopped up. The only way to know is to remove the radiator and have it flow tested.
A blown head gasket or cracked head will cause steam to come out of the exhaust, water in the oil (milky). If it is running hot while idling , check fan to see if it is turning as it should when it should. Older cars have fan clutches that go bad, newer ones have temp thermostats that go bad.
If the system fills via an expansion tank, I would recommend removing the top hose from the radiator and filling the system by pouring the coolant in through the hose until it begins to flow out of the upper radiator port. Reconnect the hose and then fill the expansion tank. That should take care of it.
Overheating can be the result of several things including: No coolant, stuck thermostat, cracked head, blown head gasket, bad water pump, cracked radiator side tanks, and poin holes in one or more hoses.
Start with the easiest things first. Check coolant level, if it's low you need to find out why. If it's at a normal level you know the coolant is not circulating through the motor. To test this run the car to temperature then feel the upper radiator hose, it should be hot and firm to the touch. Next feel the lower radiator hose it to should be hot and firm to the touch. If it's not the coolant isn't flowing through the motor which is the result of a stuck thermostat.
If the thermostat checks out ok crawl under the car and check all the connections between hoses/motor/radiator. Look at the side tanks of the radiator. If there's a carack in one they will only leak while the system is under pressure. Next check to make sure the water pump is circulating the coolant and not leaking. A siezed water pump will usually stop the pulley from turning resulting in a broken belt.
NExt look around the area of the head gaskets for signs of leakage, again this will only leak while the system is under pressure. Check the oil, if it looks milky there's coolant in it which is a sure sign of a gasket failure or cracked head. Good luck.