I was having a lot of issues with my MB8611 modem too, but I realized the problem was due to too high pwr in downstream channels. It was solved (but not completely) by adding two splitters in the line, so the pwr of QAM256 channels are dropped into the -10 ~ 10 dBmv range (except OFDM PLC).
Now the connectivity is sort of stable, but the modem still reboots every 1-2 days. I suspect OFDM PLC is the issue because it still gets 14.4 dBmv. I'm hesitate to add more splitters, because I noticed the upstream pwr has increased a lot after two splitters being added. Now it's close to the 50 dBmv limit.
Please advice. Thanks.