I'm having exact same problem on my h2h........Think I have checked about everything up to now, I have read the above and its helped me loads, at the moment now I have my cable coming in from the outside then I have my splitter, one to box then other to router then the attenuator in between and my signal has gone up slightly. it now reads Signal 77% and AGC 11%
The freeview channels are going good now without any breakup and other HD are fine but others are still blank?
Not sure whether to try a 1 way signal booster and see if that helps??