FTTC 80/20 small increase over 40Mb and gradually falling speeds

[#1]

First - thanks to PN for the opportunity take part
On 40/2 (Fibre Value) I was seeing 37dl  1.8ul - pretty much as expected. OR engineer suggested line should be capable of 65Mb+ downloads.
Cabinet is about 150m from the house.
Went on the trial mainly to benefit from faster upload speed so wasn't too disappointed to be getting in the region of 60Mb download  15 upload.
However, this is gradually declining and is now at 46.9 and 6.4 (BT Lite tester). 2 days ago (Wed) it was 54.33 and 7.9
There is no apparent difference between wired (gigabit) from the router (Belkin F6D6230) or wireless (5GHz 802.11N)
Is anyone else having this kind of problem?

[#2]

There is a tendency for the speed to reduce after the first couple of days, but at 150m you should be doing a lot better than that (I am currently getting just under 70Mbps at 450m). It is impossible to say whether the results you are seeing are due to the vagaries of speed testers or due to a reduced synchronisation speed of your modem. I get much more reliable results from speedtest.net than than the BT testers. It would help to see what your actual IP profile is, restart the router (not the modem) then run the full BT diagnostic tester. Ignore the result, just look at the IP profile. Alternatively you can get the IP profile rather more quickly here.

[#3]

@KevinG.  Thanks for the info. IP profile is:
The Service ID is: BBEU00899444
This is a 21CN line.
The current Downstream BRAS rate is: 46.71 Mbps     
The current Upstream BRAS rate is: 20 Mbps
Plusnet profile is 40Mb
Modem locked so can't get stats from there (ECI modem)
Speeds over the last couple of days look like this:

Date         08/08/12 08/08/12 09/08/12 10/08/12 10/08/12 11/08/12
Time         17:00:00 17:15:00 10:30:00 18:20:00 20:45:00 12:00:00
Download 54.33         52.25         49.78         46.94         47.56         46.44
Upload           7.9         8.35         5.61         5.97         7.87           6.49

[#4]

Well thanks to the rather precise relationship between IP profile and sync speed, we can tell that your modem is synchronising at about 48259kbps. This is very low for your stated distance from the cabinet and the decrease over several days suggests that DLM keeps knocking you down so must be detecting errors on the line. It's probably worth asking Plusnet to take a look at this.

[#5]

Thanks again, @KevinG
Have raised a ticket - Someone at CS has sent a SMS message to say they've found a 'potential cause for the fault in the exchange' so they've 'raised the fault with our suppliers for further investigation'
Here's hoping 
Terry

[#6]

I gather that since the line has now stabilized - the speeds I'm getting is about what I'm likely to get.
Still - it's a heck of an improvement on ADSL 
Thanks for your input, Everyone.
Cheers,  Terry