I've found that my filament behaves in a similar manner, as if there is a contaminant entering from the filament end. It seems to be necessary to do as you have done, snap off pieces until the snapping ceases. I've found that if I leave a spool on the machine for any extended period, it will snap on its own, just sitting there. It's not really amusing to hear, but I've come to recognize the sound. Just today at lunch, I heard my current spool, out for only one day make that characteristic click/snap and it's broken a few inches clear of the extruder. Your reference to lack of vacuum is a key. It's not out of the question that the filament was on the shelf for some time and the vacuum is no longer. If you can snap pieces to get to non-snap filament, you'll probably be okay.