Download Speed Substantially Lower Than Expected (17.4 Mbps vs 50 Mbps)

This morning I noticed that I am not receiving the 50 Mbps download speed I expect.  I have tried both speedtest.net and http://speedtest.comcast.net/, both of which report download speeds below 20 Mbps.  I have seen speeds as low as 4.5 Mbps reported -- prior to posting this Xfinity speed test reported 17.4 Mbps.  I have checked the settings on my router and all ports are set to 1 Gbps with Full Duplex.  I know I was getting over 50 Mbps when I first signed up, but not sure when the reduced speed took effect.  

My upload speed has remained steady at roughly 10 Mbps, which is what I expect.

Please let me know what else I can try to resolve this issue.

Thanks!