Its a noise issue. The SnR is dropping.
Do this for me.
Go to your router config page, and look for ADSL statistics or something like that. Somewhere there will be SNR statistics which look like this
If your SNR is falling(<15-20dB), then remove the splitter and connect the phone line directly to your router/modem.
If that increases snr, then you need to change splitter, if that does not, you need to bang your head against BSNL folks.
Due to headbanging I shifted to Airtel almost 2 years back