I've been experiencing the same thing. It's been getting steadily worse over the last couple of years. First thing I did was to get a local aerial installer to come round and test the signal strength from the dish. He said there was nothing wrong with the signal but my Sky box was so old that it could do with replacing. I bought this one for Ł100 and the problems have all been resolved.

Incidentally, that site has a huge trouble shooting section that explains in detail what to look for when trying to resolve problems.