Ummm. A bit of a strange one.
The easiest one is that you are definitely on interleaved, whatever support say, as apart from the obvious "Interleaved" that the router says, you have FECs which are the sure-fire way of knowing. They cannot occur on Fast Path.
By the way, ignore that 2147480000 CRC count, it is a known reporting bug in the Hub.
So looking at the first set of stats, the error counts are surprisingly low for a 7-day connection time. Except for the error seconds averaging 10 per day.
That is high and could very well be the reason the system is turning interleaved back on. As geordiekris said.
Looking at the second set, it is clear your connection-time noise margin is set at 12dB. This again is the result of earlier instability, and is what is actually reducing your connection speed, by between 1.0 and 1.5Mbps.
In an earlier post you said
"On the 28 it was back to 2844 kpbs and it is still at about that figure today...All these figure come from my speed log on this site". I think that means that all the figures you give are actually speed test download speeds, which is always a lot less than the connection speed. The two connection speeds in those stats are 3776kbps and 3840kbps.
Your maximum download speed is determined by the IP Profile, which is in turn set by the connection speed, but can take days to adjust upwards. Almost straight away when going down

. See my
BT IP Profile page.
You'll also find the BT DLM and Noise Margin pages useful, plus the High Noise Margin and everything except the
Router locking up ones in the Troubleshooting section.
Over a long time that high connection time margin should go down if the line is stable, but it looks as though it isn't. (BT can in fact do a complete retrain on the line which fixes it quicly, but while their seems to be instability I don't recommend that for other reasons.).
But what is odd about the first set of stats is the low error counts for 72 error seconds. That baffles me for the moment. With luck someone else will suggest an explanation.
Do you do a lot of gaming? If so, then interleaving does increase your ping time. But for any other purpose it doesn't slow you down, which is why the default setting is (I think)
On on BT.
Have you got a Sky box? That's my hobby-horse explanation for instability. But usually causes high error rates as I said.
While I and hopefully others think, I suggest you download RouterStats Lite
from here, (scroll down the page past the full version), and put a shortcut to it in the Startup Folder. Set it to save the graphs and also to log to file, and I set the sampling interval to 5 seconds.
That will let us see how stable or unstable the line is, as it shows us what the noise margin is doing and keeps going through disconnections.
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