Sync speed will drop as more and more people get ADSL on their lines, on the bundles of cables in the street that were never designed to prevent crosstalk at any more than voice frequancies.

If there is a power outage and I re-sync the router when there is stuff all other people online, I get about 50% more speed. power comes back, everyone else comes online and my noise margin drops to 0 and everything grinds to a halt on the VDSL, the ADSL line hangs in there with a 0 noise margin and runs better than the default of 12 that it will resync to.

Look at how many more wifi networks have appeared since you had the 18 meg sync. Each of those is probably another ADSL2+ connection that was either not there, or was only ADSL1 back when you had 18.

Short of replacing all the copper in the street with cat3 or better, there is nothing that can be done to resolve this crosstalk problem, and that would be stupid with fiber coming along.