This is a very common problem with this chassis.The set has a safety protection system that will shut the set down (standby)in case a fault occurs.
However,in many cases this can also be due to "dry" soldered connections in the power supply stages,particularly C12,adjacent to the main smpsu transformer
It's well worth checking this,but make sure all power is disconnected before delving inside telly's ,they are dangerous places!!!
If in doubt,take it to a B&O service agent,it's well worth repair