I had really bad sky channels in the bedroom and thought it was my tv link. I had sky 1st box installed recently and asked the engineer if he might know the problem. He explained it may be due to the change from analogue to digital and changed the RF from code 60 to code 30 on the sky box set up ( RF channels run from 21 to 68 when tuning in the tv). He then suggested I try to retune the tv ( on analogue setting) . Retuned the tv upstairs and hey presto cracking picture. Only a suggestion
So it's not actually "poor picture quality" - it's break up of the picture.
In which case I'd suggest the cause is a poor signal on some frequencies, for which the possible causes are well documented on various sites such as this: http://sam-radford.me.uk/
It could also be local interference, so read about that as well.