FTTC speed gone down understanding DLM and IP profile

[#1]

Hi
I've had plusnet FTTC since early February.  I've been extremely pleased with it.
From the installation date till late May my ip profile showing on my plusnet connection settings account and BT wholesale was around 77.xx mbit. down and 20mbit up.   When the engineer tested my line he said he could sync at 89mbit down 40ish mbit up.
At the end of May it dropped a few mbit without me noticing any drops or disconnects.   I was still getting around the same throughput on my connection though.   Then two nights ago around 2am my internet dropped for a few minutes and when it came back up I'm down to 70.2mbit ip profile and I seem to be synced at even lower.   Also I seem to have some interleaving on the line as my latency has gone up from around 12ms to 20ms.  
My main question is , Is there anything I can do to try and get my ip profile back to what it was and get rid of interleaving ?    I was a member of UKonline and BE there after that for many years and on both I could set my own target SNR and fastpath options either with hacked firmware on my router or via web controls.   I know this is not possible with DLM (this reason put me off ever having a BT wholesale ISP and almost stopped me from getting fibre too)   but no point to whine about that,  I would just like to understand what I can and should do to improve my line settings, for example will re-syncing my connection lower my profile further still ?  Should I have it for a week  then resynced it , should I keep a eye on the bt wholesale diagnostics information and wait for the IP profile to rise before reconnecting.
I have opened a ticket in relation to this speed change but I'm guessing its not enough of a change for anything to be done about it.     I'm getting 7.5mb/s throughput now when before it was 8.2-8.4mb/s.  
Thanks for advice
Steve

[#2]

Tell me about it. A nice stable 37MB then a few drops and bang I'm down to 10MB. engineer comes out says no fault found resets DLM and goes. Back up for two days then bang down again twice. The underlying reason for the drops no one seems to want to find out. See if this "triage team" can sort it. But not holding out any hope.

[#3]

A gradual lowering of sync/speed/IP profile on FTTC over months is almost certainly cable cross talk.
This is the effect of other users on your cabinet fed from the same cable switching over to FTTC and the presence of their signals on their line now interfereing with your signal in your line.  All due to the proximity of the lines next to each other in the cable.
It appears as more noise on the line so the modem has to lower the sync speed.
Cross talk affect FTTC more than ADSL due to the much higher frequencies used - hence all the many issues of this problem being raised on the FTTC service while it was a non-issue on ADSL although it was there to a smaller extent.
BT are currently trialling a technical solution to the worst effects of cross talk known as vectoring...who knows what the results will be.
ISP's have no control of interleaving on the FTTC service unlike the ADSL service.  It is under the sole control of the DLM.

[#4]

What's this fast path malarkey?
Edit - sorry for hijack.  Feel free to delete my comment

[#5]

@Oilfran, I shouldn't think anyone minds you asking the question here.
Fastpath is a term used to mean no interleaving.  Interleaving is error correction which is generally switched on if uncorrected error* rates are too high (automatically be the DLM system), this reduces uncorrected errors at the expense of a slight delay (increase in latency) - most typically 7 or 8 ms each for downstream and upstream though values can vary as different amounts of interleaving can be applied, interleaving can be applied to downstream, upstream or both independently on FTTC.
*Uncorrected errors are bad, too many means a lot of re-transmission of data (because they're uncorrected they have to be discarded), too much retransmission slows real performance down, sometimes very badly.
Back to the OP, x47c has given a good explanation, many FTTC users see a drop in speed with increased latency a few weeks or even months after initial install, this is normally attributed to crosstalk since this is known to degrade performance.  Obviously other sources of line errors could also cause the same.  Lines have been known to drop performance after a few weeks, remain at the lower performance for several months then suddenly go back up closer to the original performance (maybe the better weather as we approach summer will improve things  ), FTTC DLM is still very much an unknown quantity and generally, short of an engineer visit, there's little an ISP can do about it.