You are right, the problem is on the Moca side. I was looking at the wrong connection for my PC. It doesn't make sense though, but maybe the Moca version on the FIOS router doesn't work well with DLNA.?
I hooked directly from my laptop to the HDHR Prime @ 1Gbps and DLNA works fine in HD. I then tried a wired connection on the 100Mbps non-moca side and it works there as well.
HD DLNA seems unusable using N Wifi - at least with the non-mimo type on my FIOS router.
Strange how the same bit rates work fine over Wifi and and Moca in non-DLNA mode.