Slow speeds on Gigablast

So I've had Gigablast for over a year and have had pretty consistant speeds (650+Mbps).  Over the past couple weeks, I've gotten no where near the advertised speeds.  It gets so bad that watching Youtube requires dropping from 1080p down to 240p to not experience buffering every second, and Netflix buffers near constantly.  Rebooting the modem doesn't help at all, as as soon as the modem reconnects, I can do a speedtest and the highest speeds obtained at under 50Mbps with just my desktop connected via Cat6 cable.  Even trying other computers connected via ethernet results in the same results, low speeds.