I was having issues with a weak TV signal errors in Windows Media Center 8. I used the registry key change from this KB article at Microsoft to fix the issue. It says that it applies to Win7, I believe that it also applies to Win8.
"The registry value WeakSignalDroppedThreshold specifies the maximum percentage of dropped packets before Media Center displays the Weak TV Signal message. If the registry value is not set, the default value is 10%. The steps in the Resolution section specify to set it to 50% (32 hexadecimal). The side effect of setting this to a higher value is that it may in some cases prevent a valid Weak TV Signal message from appearing while watching Live TV in Media Center. However, in the case of a valid weak signal it may be obvious to the user, due to dropped frames or other video artifacts, that there is a signal strength problem for that particular channel."