Techwood freeview STB dv925b switches off TV. Why?

Had my set top box for about 18 months now. It started out brilliantly but now seems to suddenly switch off the TV (an old Grundig cant remember the model) to standby mode right in the middle of watching a programme. The STB itself remains on (green light showing). When the TV is switched back on it works OK for a while and then the problem reoccurs. On each occasion the interval between it going off gets shorter and shorter. Help! How can I fix this? Have tried unplugging and restarting. PS not at all technical so laymans speak please!

Chantal (North London)
January 2011