Yep, I understood all of that before replying.
Although you have confirmed that the Ceton does the heavy lifting, and the problem happens even though the Ceton has records correctly before, during, and after the problem has occurred. No doubt the Ceton uses more power when more tuners are being used. Possibly there is a fan in it that comes on or speeds up when it gets warm?
This all sounds consistent with power supply problems; either insufficient power, or poor quality power. Obviously I can't say that power is the issue, but at this stage, I would certainly be trying a better or at least newer power supply. I can't find any specs for the supplied power supply, but if it is say a 2 Amp unit, I would try to buy a quality 3 or 5 Amp unit of the same voltage output to see if the problem goes away.