Vigor 120 & RX CRC Errors

I have recently been troubleshooting an ADSL problem for a friend. He has a standard ADSL2+ connection split at the wall by a Vodafone supplied filter.

Recently, he noticed his connection dropping and the phone began crackling while in a call. The crackling is only heard by him, not the calling/called party (which I thought was weird)

Initially I figured it was a faulty filter, given that the problem was new, so we replaced it but the problem persisted.

Next I checked for faulty equipment in his house. He only has one jack point so there were no sneaky phones plugged in directly. He doesn't have Sky or a monitored alarm either :\

I removed the filter and modem and plugged the phone directly into the jack and called 120. The call was crystal. The modem! I hooked him up with a Vigor 120 a couple of years ago and it has been rock solid since then but I figured maybe two years was its lifespan...

I checked the modem admin portal and sure enough, saw a hew hundred recorded RX CRC errors. I watched it for a time and they would build up (sometimes into the thousands) then the modem would lose connectivity and reboot to begin the cycle again.

I went home and grabbed my Vigor 120 (known to be good, never dropped connection and I work it hard) and dropped it in (I used my AC adapter and cables) and saw identical behaviour.

I don't have another spare modem on hand to test (I'm waiting for my brother to bring his down) but does anyone know what might be causing this?

I have also tested running just the modem -> filter -> jack and still see RX CRC errors popping up as soon as the modem reboots.

Modem stats are as follows:

Mode: ADSL2+(G.992.5)

State: SHOWTIME

Up Speed: 977022

Down Speed: 14642423

SNR Margin: 11

Loop Att.: 18

The stats are pretty good for ADSL2+, I have seen the SNR Margin & Loop Att. fluctuate some but can't reliably reproduce the fluctuations.

TIA for any insight.