Do you by any chance have a dishwasher or something that uses hot water scheduled to run at this time? I discovered with my boiler when it is really cold, if I am running hot water for a shower or something, its not heating my radiators during that time. During our christmas cold snap when it was like -1 outside with high winds, in the time it took 3 people to take showers we lost 10 degrees inside. As soon as the showers were done temp started clawing its way back.