LOL re the TV.
On the "up to 8Mbps service", the expected difference between a sync-time 9dB and 6dB is 500kbps-750kbps. On ADSL2+ it can be as much as 1200kbps. It all depends on the line itself, and many factors.
Re the expected speed, see
this graph. Given optimised wiring, which you seem to have, I would expect to be near the maximum for the attenuation. Given that the graph is from real-world reports, even the average figure is poor, and must reflect something not as it should be.
Thinking about that, a drop doesn't need to be caused by something on your premises. It could be a neighbour, or cross-talk noise from other lines on your cabinet, and so on. I lost 200kbps when the chap across the road went from dialup to broadband, and another 500kbps when my e-side pair was swapped due to complete failure.
My broadband basic info/help site - www.robertos.me.uk
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Internet connection - IDNet Home Starter Fibre.
Live BQM.
"Where talent is a dwarf, self-esteem is a giant." - Jean-Antoine Petit-Senn.