It's a well known issue with Fios and it is the fault of Verizon (if you do a search you will see many other threads about this issue). Basically Fios doesn't have the backbone to handle everyone streaming Netflix (other services, such as Steam are also affected).
Verizon is lettering the connections that provide Netflix content (primarily from Cogent) overload so that everyone gets a degraded streaming quality. The sad part is that there is an existing free fix to this problem, but Verizon doesn't want it.
The best fix for this issue is to cancel Fios and switch to a competitor. A secondary fix is to pay for a VPN and stream your Netflix through that.
Otherwise we're all being screwed by Verizon.