It's just a long shot but sometimes the software inside these things (as in computers) just gets upset and fails to work properly. If the box has a factory reset I'd try this first followed by a re-scan. If it's still no go I'd check to see if there were any software upgrades to download, follow the dtg link I posted.
You could also try Digital Spy forums and see what's on there. This is a link to Phillips dtr 1500 search
http://www.digitalspy.co.uk/search/ds-search-f orums.php?query=phillips%20dtr%201500&page=1&s earch=2&lnpp=
One other thing I've thought of is to check the signal strength meter. Any channel below 40% will show blockiness and below 30% you'll get no signal.
The few channels I have probs with are film4 (usually during a good film too ) the two music channels TMF and The Hits ( not bothered about these) and sometimes FiveUS