My car is overheating and using up the water in engine coolant reservoir when i refill

Not fully conversant with this engine but generally causes of overheating are the following;
not enough coolant in radiator
sticking thermostatic valve
blockage in cooling system

Make sure radiator and reservoir have enough coolant, if reservoir / radiator don't get hot when engine is running then there is a blockage or the thermostatic valve is faulty.

Flush the coolant system with appropriate additive, refill and try again (you can for this use plain water without antifreeze But if it works you MUST ADD anti freeze afterwards.

If flushing system does not work you will have to replace the valve. Unfortunately I have no idea where that will be on this engine , apart from it being near the top of the water cooling systemand directly before any hose going to radiator.

Hope this helps

Check the water pump and see if coolant is around it.

There is either too much coolant in the system, or the system is overheating. The reservoir should have a "FULL COLD" mark where the coolant should reside when the engine is off and the coolant is cold. If that is correct, and the coolant still overflows when the engine is hot, you are overheating and need to check the radiator for obstructions, the coolant fan, water pump, and thermostat for proper operation.

You may have a blown engine head gasket. Check your oil to see if it is milky because water is getting into the oil, and check the water to see if oil is getting into the coolant.
If you have either of these, you may need to have a new head gasket for the engine, and hopefully you do not have a cracked engine head.
This will require a fair bit of work to fix correctly. Your local garage or mechanic may be your next call.
There are some products you can use in the coolant for either of these, but it is only a temporary fix.

The probable reason for engine stalling can be overheating.
The engine can be overheated due to many reasons and their remedies are listed below:
1. Insufficient coolant and water in radiator - Refill the coolant and water in reservoir
2. Radiator fan not working as needed - Loose connections, faulty fan
3. Damaged hoses and pipes - Replace hoses and pipes
4. Clogged pipes - Clean them

First there is but one water pump, next did you bleed the air out of the cooling system as directed in the factory manual? If you did then the engine may have a leaking head gasket. Water pump picture below. You may also have a leak in the recovery tank, but that would not cause the overheating unless you let it run dray. This engine is very easy to damage if overheated, think you should let an expert look at this problem.

Sounds like to me your mechanic is on the right track by having the rad cap changed. If the coolant becomes low again in the near future and there is no visible fluid leak on outside of engine or on floor/ground then your problem is internal and possibly a head gasket.

check the thermostat good luck

The overheating is caused by the sludge. It is very easy to correct this problem. Remove the radiator cap and open the drain valve at the bottom of radiator. If possible, remove the reservoir and flush out with water. Make sure to get all sludge out of reservoir, then reinstall. Once the radiator stops draining, insert water hose (without nozzle) and turn water on. Keep the drain valve open. Run water through radiator until it runs clear. Once it runs clear, start the engine and let it run while keeping the water running through radiator. Allow engine to run until the thermostat opens up. Continue running water through radiator with engine running until the water from drain runs clear. Once you have done that, turn off the engine. Close the drain valve and refill radiator with proper mixture of coolant and water. Once radiator is filled, start engine and let circulate through engine. Coolant level will drop as the coolant starts circulating. Shut off engine and top off radiator. Fill the reservoir with the required amount of coolant. Testing the coolant at least once a year will help prevent this from happening again. Coolant should be replace a minimum of five years, less if coolant bottle specifies.

it seems like the radiator of your car needs attention. check for coolant level in it, when its cold. if les add some coolant and then see if the problem persists.
--than there are chances of chocking in the coolant pipes, may be they not reached to engine..  
--Check the chock,, that it is not turned on. 
--May be sum loosing nut in engine.