Pixellation is caused by a signal fault, if only one of your boxes are affected this rules out an external fault.
Try swapping your boxes around - that way, you can see if the fault follows the box move or remains with the internal cabling. I suspect, if you're saying a reboot solves this temporarily, the fault will follow the TiVo.
You can then report your fault to VM. They should arrange a tech visit and swap your TiVo out for a V6, assuming you have VM's home broadband.
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