said by Ping :
hm your upstream is 38dBmv for power level which is a little low but not too far from 40dBmv which is a lower end of the recommended range for upstream power levels. I don't think your having a noise issue which would make your upstream channel change to only QAM16.
I think your problem is an over utilized upstream channel (overcrowded) I think channel bonding would help possibly if your node is not maxing out on all of the channels. My area is bonding 4 upstream channels right now. I would assume that upstream bonding will come to your area sometime this year or next.