Toshiba fault finding?

I have a Toshiba 19inch Digital TV in my bedroom but from time to time I get the message that no signal is being received. The problem is that this can apply to most of the programmes or just one. I have an outside aerial and the two other televisions in the house are receiving a signal so why does it happen with just this one. I have from time to time had to rescan but this loses me all the carriers and it can take a few days before I start getting them back one at a time. Any advice would be welcome. Thanks