HD DROPS OUT ON SOME CHANNELS IN AFTERNOON

For the last couple of weeks every day at about 4 pm we get the searching for signal error on the hd channels of USA, SYFY, TNT, and TBS. These are the ones we watch at that time of day, so it may be others too. Our local hd channels work . There isn't anything blocking the dish, and we've had no storms to interfere. The non hd channels work fine. Sun spots?