channel tuning?

I have 3 thompson freeview recorders2 DTI6300(160G)
and one just bought with 250g, the first 2 tuned in ok
but the third one for no reason has the channels listed all over the place like bbc1 will be channel 875 and itv will be way up the list at 812 which to say the least is a pain to search through the channel, the manual does not seem to have a answer to this, anybody have any ideas
regards
dave