A migration from TiVo firmware to Horizon/360 firmware is a one-way trip. Some people have managed it, but only with some significant effort and a very hard-fought battle. For all intents - you can't revert.
Having said that - buffering when streaming sounds like a connectivity issue to me. Not necessarily a 360 issue but probably fixable. Can you explain the symptoms a little more? Is this VoD? Streaming apps? Multiroom streaming? And how are the boxes connected to your homehub - Ethernet or WiFi?
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If you want to pause for longer than 5 mins when using the mini box you need to set the programme to record on the main box, easy enough to do just press record on your mini box and it will record on the main box and then you can pause for longer than 5 mins. Just make sure both boxes are set to fast start or active start or it won't work.