Check your stored tuner channels for premium channels. If one of your 4 stored channels is a premium it can cause the problems you indicate. So when not watching TiVo ensure your 4 tuners are not left on premium channels. Sometimes this can't be helped if you have a premium channel recording scheduled.That seems to be the issue! Due to our cable provider's locally archaic system, we have to have a A2D converter in line on the video feed to the TiVo. A reboot of that seems to have cured the problem. At least for now!
Given the costs these days of the "Kind-of-Sort-of" For Life (Of the box) service, I'm happy it's still getting along. I'd love it if TiVo offered cloud storage for emergency back-up.
As always, this community always provides some great insights into any issue!