This one has me baffled. For the last several months I've been getting sporadically high RS corrected/uncorrected errors on a few channels resulting in pixelation/macro blocking. The odd thing, is the sporadic nature of these events. Most of the time I have zero corrected/uncorrected RS errors on all channels. However on a handful of channels I'll notice pockets of these errors. I had a senior tech come out to check my levels and he said they're perfect and that the average error rate was one in 9 billion which he said was very low. I've noticed this behavior on two Premiere XL4's as well as my new Roamio Pro - so it's not limited to one particular TiVo or even one cable line. FYI, I have no splitters only a zero gain amplifier.
At this point I can only speculate that it's either a condition that is accentuated when under load, or the errors are in the transmission stream itself.