I guess that means you're in the Granada region. I cannot remember the channel number from Winter Hill for Channel 5, but my guess is that the signal for that one is somehow a bit poor in your area... I'm south of Chester and know that the bulk of signals from Winter Hill are in the upper 50s and 60s, so no aerial change was needed.
Has the aerial been moved in strong winds? Just wondering if it's a signal strength problem. Some Freeview boxes show a "quality" score out of 10 and if it goes much below 4 I had problems (with a Thomson box).
Have had no TV for 2+ years and cannot test anything right now. Anyway, if it is settled a bit, hope it stays stable for a longer while!
The Wiki page about the Winter Hill site says that Mux A (UHF channel 68, highest frequency in use at Winter Hill) is at 5 kW, compared with the other digital services using 10 kW and the analogue signals from 12.5 kW (Five, why it was a bit grainy in N Wales) to 500 kW for BBC1/2 etc.
Apparently when they switch to digital the power will be pushed up a bit, and the total output will change from 2000+ kW down to under 100 kW.
Looks like the broadcasters' electricity bills will drop substantially!