We reproduced this issue with an Entone Janus PVR and an Entone Amulet HD at the customer’s house. Everything like wiring, modem config, checks out fine.
We are able to watch something fine on WHDVR however when we fast forward more than 10x the speed the picture completely freezes or goes black which doesn’t allow you to see where you are.
It does not seem to happen on a Entone Amulet 455 PVR over WHPVR with an Amino STB.
Update January 25, 2016 12:20PM: Confirmed I have reproduced the problem with a Janus PVR and Entone Amulet HD STB.