since yesterday I am experiecing a rather strange problem with my mining rig.
The rig is stable for a couple of minutes, then all of a sudden, one card just crashes and the phoenixminer shows the following error :
hwmc GPU1: unable to get temperature - GPU is lost (15)
hwmc GPU1: unable to get fan speed - GPU is lost (15)
The card in HWinfo is still shown, but in a bit more light-gray-ish colour, as if its grayed out and not in black, like the rest of the GPUs.
I am currently using Phoenixminer 5.5c and the latest drivers of nVidia(this was happening with the old ones too)
Can anyone help me

Driver failed. Maybe bad riser, bad cable for riser, card dead. Try without this card and then try this card alone. Thats always the first things you can do.
And by the way, some information about your system is maybe helpful for other people to help you.