df2334:[snip] Especially that SNR. 

Oops, missed that comment first time around. SNR is a target set by the DSLAM, in your case, it's set at 6dB due to being on non Chorus gear. When the modem syncs, it will aim for a 6dB noise margin and adjust the sync rate to get this. The noise margin should remain static, maybe varying +/- 0.2dB or so. If it is fluctuating significantly it means there is noise induced on the line that it coming and going - an SNR of more than 6dB means that the background noise level has dropped since the modem last synced and vice versa. Phones, alarms, Sky decoders and the like can introduce this periodic noise if there is no master filter, however external sources are also possible - your feeder runs past a factory that's machinery causes interference at certain times of the day, or water/temperature affecting a cable joint.

Basically, whenever any brand modem syncs on that line, you should see very close to 6dB - it may then vary over time if the background noise varies.

Don't waste money on getting a new modem yet. Get your wiring sorted, it like there could be several issues given the fluctuating attenuation, SNR and FEC errors in a short time. Once the wiring is sorted, see what sort of ADSL connection you have, and you'll be in a better position to judge what VDSL you may get.

Stick a few of your neighbour's addresses in here https://www.chorus.co.nz/broadband-checker to see what sync rates they are getting - if they are significantly higher than yours, it's pretty much guaranteed you have an internal wiring issue.