Had anything been done to the vehicle just before the problems? I wonder if something corrosive got into the water? Possible sources: adding liquid, adding miracle item to cooling, having radiator system flushed by commercial operation (they never get all the cleaner out and it is very corrosive) using a flush and not getting it all out (you must flush with clean water a whole lot more than the instructions) using water only without at least 50 percent good antifreeze(especially if you use "hungry" water such as distilled or ozmose) If you had a repair shop do the work, don't let them work on it again.