That's all the obvious things then. If the AV, HDMI or TV was causing this, then taking it out of the equation should identify it.
If the TiVo is causing it, a reboot should solve that.
If neither apply, then it points the finger at a possible box fault. Given that a box replacement will lose all your recordings anyway, you can try working through the graduated levels of reset under Home > help & Settings > Clear or restart.
Each level of reset has on-screen guidance as to what will be lost - otherwise you can wait for forum staff to pick this post up which may take a day or so. I can't imagine their remote diagnostics would pick up anything that a bit of "investigative troubleshooting" won't - but you never know.
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