I had the exact same issue with my line syncing at 9934 Kbps / 438 Kbps when I used
BSNL on the 2 GB CUL plan.
My modem was configured to run on the d.Gmt mode for better sync instead of the
ADSL 2+ mode.
The highest I ever got over various speed tests was 6.76 Mbps on download.
Far from the rated speed of 8 Mbps. This despite a clear line with low attenuation
rates of 12/8 on the downstream/up and a SNR of 18/31 on the downstream/up.
I tried ignoring the ad injection and the redirects for a couple of years, and despite
being friendly with the folks at the NOC, NIB and the exchange, they couldn't
do better. I was advised to switch to BSNL FTTH after getting 5 more users.
All in all, I surrendered the broadband connection while retaining the phone line
this June and switched over to a FTTH connection 50 Mbps (symmetrical) from my
local cable provider. I still have a backup unlimited airtel ADSL 8 Mbps connection
which gives me around 8.8 - 9 Mbps download speeds.
Things are good now. Consider if you can switch to an alternate rather than breaking
heads with BSNL. My BSNL connection was installed in March 2006.