I have three TVs connected to a distribution system with DirecTV and a diplexed OTA antenna which resides in my attic. The living room flat screen (where the multiswitch resides) gets reliable reception of 23 channels. One bedroom TV (non-HD) gets reception on 27 channels after a 60-foot coax run and a diplexer. The other bedroom TV (also non-HD) gets reception on 33 channels after a 70-foot coax run and a diplexer. I tried an amplifier between the antenna and the multiswitch with worse results. The multiswitch has an OTA input, so I ran a coax from the antenna direct to it with no change. I live outside (30 miles) of Houston in flat, virtually treeless terrain. Also, i have a small flatscreen in my computer room which does not connect to the DTV system. I have tried four different indoor antennas and every time someone walks into the room, the reception blanks out, pixelates, and eventually comes back on. I never had these problems with analog OTA. Digital Hoovers. Any suggestions?

It's possible that the signal for the flat screen is too strong on some channels causing distortion on other channels that are near by.  For example if you have channels 23,24,25 and they are all quite strong they can cause distortion on each other and none come in.  The 60 and 70 foot cable runs reduce the signal enough that they don't interfere with each other anymore, which is wny the longest cable run gives the most channels.  Here is something to try, move the living room TV to the bedroom with the most channels and see if it comes in better there, if it does then you have your answer.