Result!
Great stuff. Perhaps Johnson will be able to give a bit more insight to the reason why.
I have no more of a clue than anyone, have just tried as many different firmwares on as many different modems as I could.
A2pv6F039i.d24e
This is both an older adsl_phy and an old kernel driver. In fact this is exactly the same version as the (now very old) latest stable firmware for the Billion 8800NL v1 uses.
So it seems some regression or new bug has happened in BCM DSL code for vectoring, in my case its half speed syncs after "hot" reconnects but in Pottsy's case its half speed or worse in all cases. From my testing this started with the A2pv6F039
v.d26a combination, any early ones work as intended, and later in A2pv6F039
x1.d26a.
If you stick around and are willing to test things Pottsy (once you have let your line settle after all these forced resyncs) it would be great to see if a firmware with an earlier adsl_phy eg A2pv6F039
o1 with the later kernel driver i.e d26a vs d24e you are running now still fixes things for your particular line.
Either or, if you would like I'm sure we can cook up a latest version firmware with the correct combination of adsl_phy and kernel driver to suit your line.
Edit: Adding that we are on the same SW version at the cab:
DSLAM type / SW version: BDCM:0xa48c (164.140) / v0xa48c
Modem/router firmware: AnnexA version - A2pv6F039x1.d26a
DSL mode: VDSL2 Profile 17a
Status: Showtime