The NG is actually a bit of a lash-up of an updating to the 300 -500 versions to try and bring the line up to near Airbus technology. Initial delight at a near 747-400 flight system becomes disappointment at what it can not do that its big brother can. You can tell the old technology behind the bright and shiny facade. The embarrasingly non-existent EICAS system, the total lack of autostart and auto-icing, the awful integration of systems that should really have been sorted out with the NG version, the ancient cockpit design- why ever couldn't they have replaced the front end with the lower drag quieter, 757 front? It really is still a pig dressed up in a DJ. They actually updated the 737 line to approximately 1985 technology....and that is what they are still selling today! And don't even bring up the 757 system! Noah had better equipment. How do Boeing get away with it? Not even a VORLOC switch! Can you believe it?
What was the question again? Oh yes- so that is probably why the ILS is prone to such external errors.