Poor Signal On Local Channels

I've noticed over the last month, poor signals on my local channels here in Phoenix.  I get pixilation so bad I can't really watch the show.  It seems to only be on the local network channels (CBS, NBC, ABC & FOX).  Doesn't matter if I'm on 15 or 1015.  Channels like HGTV, HIST, SCIFI don't seem to be affected.   It starts every day around 11am and seems to stop around 6 or 7 in the evening.  My suspicion is it is heat related but then what do I know.  I just know my wife is not happy!