I too have the same problem. It started on the weaker channels but now incudes the BBC.
I'm not sure what you mean by "weaker channels", all channels are carried down your co-ax at the same power levels. And they are carried in "clusters" that don't necessarily bear any resemblance to the EPG positions.
Engineers are like any other walk of in life, some are very good others not so good, you certainly don't need to spend the silly money quoted, japitts is perfectly correct there are no "weaker" channels, sounds like you have been unlucky with the engineers you have encountered.