The short answer is, no you can't.
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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