hi,just had this problem on a vestel set.the cable(flat or round) that goes from the main board to the actual screen is normally the cause for this.the screen end may well be under one of the pcb's.they normally look as though they are inserted ok,but are not.if you can open the back of the set up support any boards that might be in the way , turn the set on and wait for the fault to appear,then gently push home the cable at the screen end.if this cures the fault you will need to cut the old glue away,re-seat the connecter,push home all the individual wires with screwdriver,turn set back on ,hold in place and glue back(hot glue gun)