well, 6dB is normally accepted as enough headroom to make the link reliable, but some lines need more than 10dB...to make the connection more reliable the snr margin should be ideally above 12dB...anything below 10dB will result in frequent disconnections and unreliable connection in terms of speed...so get ur line checked by a linesman and get it redone...alternatively you can change your modulation type to "multimode" or "g.dmt"...these modulation modes show better snr margins as compared to "adsl2+"...