SNR on downstream of 28.9 is quite acceptable, indeed downstream remains acceptable just about until 23.5dB or so, the ideal is 30 however this isn't essential and BER remains workable at a much lower level than this (at 23.5dB BER is around 1 x 10^-8), 30 is required for the more strenuous 256QAM modulation which isn't live on ntl network at the moment (at 30dB CNR BER is 1 x 10^-8).
I'm suspicious of your cable modem to be honest and would suggest that gets looked at firstly, your SNR although not ideal is within spec and should be quite acceptable.
Can't see any sign of ya IP btw Tuftus
