This appears to be the continuation of the same issue you recently started another thread on:
Double posting leads to confusion -- all the discussion of a particular issue should be in one place.
You have bizarre signals that behave in a way I've never heard of. A signal level of 86 and SNR of 34 dB should give you excellent performance with NO RS errors of either type. I suspect some upstream device (e.g., a QAM modulator) is being overdriven and is creating errors in the signal. Whatever the cause, the problem is in your signals and would have to be fixed by the cable co. I would challenge them to hook up one of their STB's or DVR's and show it performs well with these signals. (If it does, I think I would consider giving up on TiVo at that point.)
There are two issues with hot signals on TiVos:
1. There is protective circuitry in TiVo tuners that will cut the signal off momentarily if it's hot enough to do quick damage.
2. Signals that are hot but not enough to trigger the protective feature will damage the tuner circuits over a long term.
Given you had to use 20 dB attenuation to get the signal below 100 I wouldn't be surprised if these issues apply.
Network error messages are new to me. The obvious interpretation is it's related to your internet connection but how could that be affected by different attenuators in the video cable? I wonder if it refers to the OOB (out-of-band) signals that the TA and CableCARD use to communicate data with the cable plant? Perhaps these signals got too weak with your heavy attenuation.
I can tell you how to check those levels if you want to get that deep into the weeds.