Why is my DTR not stopping recording at at the right time?

In the last week, myThomson DTI6300 DTR has started to keep on recording well past the time it should have stopped, either recording for over 3 hours everything on one channel & nothing on the other channel, or recording for over 3 hours on both channels when it should have switched off after 2 hours. The only way I've been able to stop it is to switch off at the mains,with the result I then lose everything I was trying to record,even though the programmes had finished. Can anyone help or advise please?