I had similar problem, it turn out being the cable and splitter. I replace the spliter with a digital spliter, I also replace the connector with digital, it work work one day then bad picture, it turn out to be cable from the unit to the spliter. Also try a differnent channel, example channel 21 will not work, but 22 is okay.
hmmm Kinda wondering about the wiring in the wall now. I put a splitter on and the TV in the living room is crystal clear but still the one in the office is not. The office has a shorter run than the living room. I also tried swapping the leg on the splitter each was connected to and same result.
Thanks everyone for helping me narrow it down and now I know I don't need an amp etc.
WOOT WOOT!!