Had a Problem, which looked like Capacity issue. ISP said errors issue and increased SNR to 12. Same problems persisted so obtained Billion 7800N
Ran perfectly with download SNR of 3 for a week with only same capacity issues.
What was your sync speed after you set, or had, a downstream SNRM of 3dB?
If it was in the region of 12000Kbps you could try forcing ADSL2 with a 3dB SNRM. You may find that the sync speed is reduced by ~1000Kbps and also the error rates may also be reduced, thus enabling a more stable line.