I started having a packet loss issue around the same date, and I also use a Fritz!Box 7490. Two engineer visits and no faults could be found; the JSU showed the line was perfect.
I heard about another Fritz user with the same symptoms, and I checked and noticed that Fritz!OS 06.80 was released around that time. I used the DSL option to use the previous DSL formware, and since then my errors have disappeared and my line rate has restored: it took DLM a couple of weeks but I'm back to where I was. I've avoided upgrading to Fritz!OS 06.83 since it doesn't mention and DSL fixes.
You may not have the same issue, but downgrading the DSL firmware might be worth a shot.
The timings don't work for me, and I did back revert to 6.52 for a day or so but the graphs still looked awful. I have now been on 6.83 for a few days so I am not convinced that it is linked to the firmware. I may be wrong so I have sent AVM a full diagnostics test.