I'd have thought that -8dBmV is within spec for QAM64 but borderline for QAM256.
And since these figures drift, it is probably going out of spec. the best bet is to post them on the support newsgroup, where an expert will give you a proper answer.
As far as Tuvete is concerned, The upstream Transmit power is too high, should also probably leave a note on the support groups.