The engine shutting down, presumably on overheat, and losing coolant are no doubt related. First shoot the thermostat housing with an IR gun with the engine running and under load to get an idea of degree of overheat. Should be no more than 200 F.
There are three areas that I would look into:
1.The raw water heat exchanger being scaled up causing the overheat and coolant blowing out. But in this case it would blow by the "radiator" cap and not out some unlikely place at the back side of the engine.
So, remove the end caps on the heat exchanger and rod the tubes with a .22 rifle cleaning rod and then circulate Barnacle Buster or similar to remove the scale.
2. A coolant leak somewhere on the back side of the engine. You can buy a dye that will make it easier to see where it is leaking.
3. A bad head gasket or worse a crack in the head letting combustion gasses into the coolant. Once enough coolant has been blown out, the engine overheats and shuts down.
This is easily tested by filling up the coolant, running the engine until warm and looking in the filler port. If you see bubbles, it is probably exhaust gas leakage.