vaillant eco TEC 831?

my vaillant eco tec 831 works fine when the heating is on constant but when on timer and the heating goes off i lose all pressure and display flashes f.22 0.0 .i then repressurise system using filling loop and all works again fine until time goes off again.i.e go to bed get up in morning its freezing cold because boiler does not have enough pressure to turn boiler back on even thou timer is set to come on.i have checked under floors for leeks and there are none up. any ideas or has anyone out there suffered same problem