Still doesn't explain why while downloading a game, or firmware update should take a ridiculously long time and a BT speed test is showing a download speed of less than 1meg.
So if I read you correctly, your line "works" until your try to actively transfer something large-ish over it, right?
If so...
My line is full of errors at higher speeds,
I wonder if this (^^^) is key.
If packets transferred over the link get corrupted, they need to be re-sent... using up a portion of the capacity of the line, and a portion of the capacity of the computer.
Once a certain portion need to be resent, the overhead of re-sending starts to be noticed. Even worse when a portion of the re-sent packets go missing too. The situation escalates at an ever increasing rate.
So perhaps your error rate is enough that the whole line grinds to a halt.
Have you tried running a TBB Ping Meter on your line?
I can rule out the line as 3 Engineers have tested it,
Except that you get a high error rate, and seem to have intermittent problems - the hardest sort to fix. It seems you have an intractable problem that is very hard to identify - so very little can be truly 100% ruled out. All that can happen (like the engineer who identified your TV as a culprit) is to identify each problem in turn, and see what happens after fixing it.
and 7 years of Sky BB didn't show the same issues.
That suggests that the problem either
- is in the wiring or equipment that was added/jointed when your line was taken away from Sky, or
- has been introduced since, or
- existed anyway but that the reaction of the different DSLAMs (or DLM) is having a catastrophically different impact.