there aren't alot of hose's on the 420's that are clumped together, their pretty seperated.
theres two that come off the thermostat housing (the big one that goes to the radiator and the tiny one that goes to hardpipes), theres one that comes off the bottom of the radiator that goes to the pipe coming off the water pump that seperates to a smaller hose that goes to hardpipe that is shared with the one coming off the thermostat housing. those two hardpipes go to your heater core houses.

OMGOSH is right tho. that is a good identification process. but definitly try to find the exact cause of your water loss otherwise buying a radiator could prove to be a waste of money.

and have you eliminated the fat that it could be your sensor? I doubt it is because it consecutively does it at a stop and not at random times.