Another thing that can cause issues is electromagnetic interference (EMI) caused by either cheap cables and/or HDMI cables being run next to (parallel) power cables. One way to reduce this is to buy ferrite cores and attach them to each end of the HDMI cable. You can get ferrite cores pretty cheap (<$10) from ebay and Amazon. Other than that, you might just have to pull a new HDMI cable through the wall. I would suggest one from Monoprice, preferably a certified cable.