Different receiving devices (whether they be set-top boxes, TV sets, PVRs or whatever) all have different 'sensitivities' i.e. their ability to handle weak signals.
If you didn't experience the problem with your previous box it means that it obviously had good sensitivity. Your new box hasn't got such got good sensitivity and is failing to cope with weak signals.
The signal strength received at your aerial doesn't remain constant. It's affected by the 11-year sunspot cycle, individual solar flares, atmospheric pressure, precipitation, interference from other electrical devices in your home (such as computers, routers, TV sets, etc, etc) and similar interference from your neighbours' houses.
Up until now the signal strength has been good enough for your box to handle but now something has changed which means that the signal strength has dropped below the threshold required by the box. (That could be from something as simple as your next door neighbour installing a new TV or moving a computer into a different room or it could just be to do with the weather).
You clearly need to improve the signal strength. Theoretically you should call in an aerial fitter to replace both the aerial and its associated cabling. However, given that the signal strength you're currently getting is apparently
nearly good enough, a simple signal booster should do the trick.
All of the big supermarkets sell signal boosters for (typically) around a tenner but this one, from Maplin, boasts better 'gain' (meaning that it's more likely to be successful):
http://www.maplin.co.uk/p/1-way-aerial-amplifier-rt67x
[That's not just derived from a bit of Googling; it's what I use!)