Pixelation

I have several of the rooms in my house that were pre wired with cable. When I prewired it I split the signal several times to get the signal to the different rooms. I currently have a digital contour box and two mini boxes connected. I get a great reception on one of the mini boxes and the contour receiver. The other mini box the picture constantly pixelates. I have a coaxel cable from wall to mini box and then a hmdi from mini box to TV. Any suggestions would be appreciated.