Could be a bunch of stuff. Bad network card, voltage being backfed over the Ethernet, a virus munching on wired bandwidth. The fact that it doesn't happen when the Ethernet is unhooked would make me doubt it's a cracked fiber.
It could be AT&T. They overprovision the circuits. A 1Gbps on Xpon will test 1300.
Could be a bunch of stuff. Bad network card, voltage being backfed over the Ethernet, a virus munching on wired bandwidth. The fact that it doesn't happen when the Ethernet is unhooked would make me doubt it's a cracked fiber.