What do you mean by lose signal?
I use hdmi on my 47" vizio and I occasionally have to unplug & replug back in my cable on the video card. Happens randomly when I leave the room for a few hours lol.
You sure you have 1080p enabled in your graphics settings? I know with my 4890 I had to fix all the scaling & allow 1080p@60hz etc etc before it would work right.