I can play for hours on end as long as I stay in the top half of the map, no crashes whatsoever, but the minute I go to somewhere like the USS Constitution or try to get to the Castle, I'm unceremoniously dumped to the desktop with no warning, "exclamation sound" or anything

The lowest I can go on the map is about the Water Treatment plant and the Boston Brewery, as long as I stay above that "Latitude" I'm fine and experience no crashes.
The minute I go below that 'latitude' I CTD