You've answered your own question!
The downstream power level should be as close to zero as possible so that under varying line conditions there is margin to fluctuate either side. So if you're seeing constatnt power levels above 3 dBmv, then something's amiss. How amiss? Have alook that this thread:
http://www.cableforum.co.uk/board/12...al-levels.html
It's difficult to tell from your stat whether there's a modem fault or whether there's an instability in the network somewhere. It might be that your coax needs screwing in tightly.
Does that help? Prolly not because your power levels are not outrageous.
Other brainios could venture an opinion too.