Have you checked it's not the VDSL2 faceplate filter?
Actually, I don't use a SSFP. I'm on distributed filtering.
However, I have changed these, yes.
There has been a change today that makes me more certain that it is the DSLAM. After a resync at ~5:30 this morning, it continued with the same ~7 min 2:1 square wave for another 3 hours before settling into something more like the normal line variation for 7hrs before a ripple re-emerges, though now at half the amplitude and twice the rate!
Go figure, but my guess is it's the DSLAM itself. Perhaps a faulty line card or PSU in the cabinet.
Still can't think of any useful reason for such behaviour.