Why is my digital tv reception very poor at times?

we have a set top box as well as digital on our tv, we have serious problems with it, at times it breaks up so badly that we can barely watch it. We have re-set it numerous times, to no avail. Could living in front of a train line have anything to do with it, or could it simply be the outside ariel. We live in a flat so access to check ariel hampered. Any help/advice on this matter would be greatly appreciated. thanks