Most likely to be a dish installation problem. Most engineers dont position the dish for optimum signal peaking. This is likely to cause problems on channels transmitting with less noise/gain ratio than others.
There was something about Setanta on TV the other day; loads of people were having problems with broken transmissions. It was due do Setanta not using the usual Astra satellite that most sat TV channels utilise and the fact that the dish hadnt been peaked for optimum viewing performance.
Getting a reputable installation engineer is the hard part, without the use of spectrum analyzers and other test equipment its difficult to optimize the signal. Most engineers simply use a compass and fix the dish in roughly the direction of the satellite. 0.1 of a degree out will make a hell of a difference. Also getting a decent mounting helps, a bit of wind and the dish starts moving will guarnatee drop-outs.