There is an electric radiator fan on your car; which is activated by a thermal switch that is located in the water pump housing. This switch can be distinguished from the sender for the temperature gauge, because the fan switch has a 2 conductor plastic plug on it; while the sender for the temperature gauge looks similar; but has a single bare metal post for a terminal.
The thermal fan switch is known to be unreliable; and it will make the engine overheat if it goes out. The electric radiator fan should run whenever the temperature gauge goes more than 1/2 to 2/3 of the way up. If the fan does not run; there is nothing to prevent the motor from overheating. So check to see whether the electric fan next to the radiator runs when the motor gets hot. If it doesn't run; replace the thermal fan switch. The first place a bad fan switch shows up is that the car overheats when idling for long periods; but cools down when the car starts moving. Once the head gasket blows from severe overheating; the engine will get hot and lose coolant all the time.
If the temperature gauge ever is allowed to go up into the red zone; it often causes the cylinder head to warp and/or crack. The head gasket will then begin leaking coolant into the cylinders and blowing it out the exhaust, or sometimes it leaks coolant into the engine oil, Other times; it lets engine oil get into the radiator. If you want to prevent expensive repairs: NEVER let the motor run when the temperature gauge is heading for the red zone. This is the car owner's responsibility!!! Stop the car, and immediately pull off the road; regardless of where it is. This may be inconvenient; or even force you to walk a long distance; but you'll usually save thousands of dollars in potential repairs by walking that distance. Is that worth it???
It sounds to me like there now is a head gasket leak; along with a warped or cracked head. You can confirm this by taking the car to a radiator shop and having them run a check for a leaking head gasket.
I expect the fan switch has been bad all this time; and was not tested or noticed; and that, along with driver negligence, may have led to damaging the rebuilt head. This is something a professional mechanic should be expected to check without prompting. But in this day and age; there are all kinds of people who call themselves mechanics.
There is only one type of chemical sealer for head gasket leaks which consistently works. It is a ceramic product called Irontite Ceramic Motor Seal. Available at some auto parts stores, and can also be ordered directly from www.irontite.com
If you decide to use this type of product; DO NOT LET ANY MECHANIC OR PARTS CLERK TALK YOU INTO USING A DIFFERENT BRAND. Even Irontite makes another product (called "all weather seal") which does not work for this purpose. So be sure to only use Irontite CERAMIC seal, and carefully follow all the directions on the bottle. This product cannot be used in anti-freeze; so the cooling system will have to be drained, thoroughly flushed, and filled with plain water before adding the sealer. After using the car for 2 or 3 days; the sealer should be drained out and the cooling system filled with 50% anti freeze and 50% water. And the thermostat should then be put back in.
Whenever a radiator or cooling system is worked on and refilled; there will be some air left trapped in the motor. This air will come out into the reservoir during the first few days as the motor runs; and that will leave the radiator level low. At those times; the coolant level in the reservoir cannot be trusted to indicate whether or not the radiator is full. The only reliable way to check the coolant level is to remove the radiator cap (only when the motor is cool) and fill the radiator to overflowing through the cap opening. Keep checking and refiling the level in the radiator EVERY DAY until the level stops dropping; or until the source of the coolant loss is found. Only when the level stops dropping can the reservoir be trusted to indicate whether the engine coolant is full.