Hmmm.
It's tricky because it could be either a fault with the TV (causing it to switch sources for no good reason) or with the digibox (producing too low a signal for the TV to recognise). It would be interesting to know whether the problem still occurred with a different input source (such as a DVD player). If it did then it's clearly a TV fault; if not, it's probably a digibox fault.
Assuming that it's a Sky digibox, I'd be tempted to get hold of a cheapie from eBay, insert your existing Sky card into and see if that makes any difference:
http://www.ebay.es/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=sky+digibox&_sacat=0
If not, you've only wasted a few euros (which you can recoup by reselling on eBay anyway) and you'll know that it's definitely a fault with the TV, so that you can get someone to look at it.