READYNAS 102 Sync to DropBox Throttles Internet Speed

I have three ReadyNAS 102 devices, all synced to DropBx, one-way sync NAS to DropBox. It worked fine for many months. I have downloaded all the latest firmware as it became available. All of a sudden my internet speed dropped from 800 MBPS to anywhere from 2.5 to maybe 100 at most. The problem disappears if I turn the devices off, or if I terminate the DropBox sync. I am not sure if for some reason I am totally saturating my Comcast bandwidth when the NAS units are activated, or if there is something about them that causes Comcast to throttle my bandwidth. Anyone else have this issue or a solution? Thanks.