We had similar problems just before Christmas. I think it is weather related - ours varied between the BBC and channel 4 & 5. We have a signal booster and found if we changed the frequency on that it improved but had to be careful as improving one channel sometimes made another worse so had to fiddle until all ok. Upstairs downstairs is on i player and the first episode shown on Sunday was good and we will be watching the second today (Boxing Day!).