sometimes i had the same problems as you and i just let it go.

What are the consequences of forcing Interleaving off, eg fast path. More sync I presume but will the line become less stable?

If a line suffers from some noise then Interleaving can help with correcting the errors. But, if errors are low then fastpath would give you higher performance and lower latency. In the end it all depends on the characteristics of the line - the shorter the line the more benefit to fastpath.

I'd agree with that. In my old house I had a line with 35dB attenuation that just managed to achieve maximum sync on ADSL. When I went to an LLU provider and ADSL2+ it was ~ 9Mbps iirc. I had them try turning off interleave which was on by default and the throughput and stability suffered so I had to get them to put interleave back on. To my mind anything less that a pretty good line needs interleave although gamers will put up with the problems using fastpath on such a line will bring.