It's housed in a brand new underground conduit running from the new building construction to the old building, and there hasn't been construction going for months, so I can't see how anyone would have cut it or anything.
Do you know what is running in the conduit with your cable? Or even in adjacent conduits. It doesn't have to be physical damage you are looking for. Has anything changed on site recently, anything new electrical installed?
A quick and easy test for this is to use a longwave radio, if you are picking up massive mains humm across the range you have some kind of interference onsite that could jump over into your cat5 wiring over a distance like that.
Just an idea, and is a simple enough check if you do have a battery long wave set to try it out with.
Cheers
Cris.