Not so much fluctuations, but rather the DLM system is monitoring and starts to slow things down to reduce errors after a day or two, and the tendency is for things to decrease until a stable speed has been found. If things improve and stay improved the DLM system does relent.
If you have been doing lots of 'testing' that involves plugging and unplugging the modem this may have helped the DLM system to think things are worse.
The fact the test socket offers some improvement suggests that in home wiring is probably picking up errors and the DLM is reacting to those. If ring wire is still attached on extensions lift it out of the IDC jaws (pin 3) at extension and master faceplate.
Extending the Ethernet is not going to cause an issue unless you use cotton for the wire or distances are over 100m.
As the DLM system has seen a few days of poor performance by now, if you do make things 'near perfect' then you may not get the speed back for a few weeks.