Hi,
The problem with digital tv reception is that in order for it to work it continually masks errors ( like CDs do ). You then only see a problem when the amount of errors become too many to correct. For this reason you will need monitor the pictures over a few minutes to make sure the signal is consistent whilst aligning the aerial.
I too have problems on the more compressed channels such as TMF and The Hits.
There should be somewhere on your system a signal strength / quality meter. Quality should always be on 100% if not the picture breaks up. Signal strength should be at least 40% for consistent pictures and sound.
With my reception BBC 1 is the best channel and it never drops below 45% but TMF is usually 38% at best to 25%.
Here are a couple of links that may help sort your reception out.
http://www.radioandtelly.co.uk/reception.html
http://www.bbc.co.uk/reception/