Sorry to hear that your On Demand keeps cutting off. Can I ask whether your TV Box is connected via a wired or wireless connection? When it happens, do all devices cut off at the same time? Please give us more information so we can help.
In addition to @Lisa_CC question, it might also be worth confirming which TV box you have - is it a V6, that relies on a connection to your homehub for its internet? Or is a TiVo with the connection entirely internal?