Disconnections and dropped speed

[#1]

My line has been stable for a few months syncing at just over 5400 with a profile of 4500.
Sometime last night (between 20:30 & 02:30) it resynced to under 4800 and now my profile has dropped to 4000.
Once again, there was no indication of a dropped line when accessing the router's stats as up time for WAN/LAN/WLAN were still showing in excess of 700 hours!
I have tried a rebooting two or three times during the day but the maximum sync is 4832 meaning my profile won't lift.
Current stats:
Up              Down
4832          448
ATT: 48      13.5
NM 9          22
Could someone from the team please have a look at why, once again, a seemingly stable line has started dropping once more?
Since moving to this property last November this is the third time this has happened and usually the NM creeps up and I have had to request a reset.
Any help appreciated, thanks.
Edit: Just to confirm the router is connected to the test socket within the master socket and the PC is connnected by ethernet cable.
Here's the last two threads I previously started regarding line issues: Constant Reconnects Noise Margin all over the place and low sync

[#2]

Hi Mav,
I can't see any obvious reasons. I'd firstly troubleshoot it and then raise a fault if necessary.
Jojo

[#3]

Thanks Jojo
I always do the trouble-shooting before contact with PN anyway.
I'll try rebooting once a day between 10:00 and 14:00 for a few days to see if the sync rate rises.
Plus I'll keep an eye on the NM which, by the way, has gone back to 6db. Tried another reboot just now but no change in sync
If no improvement by next week I'lll try raising a fault.
Cheers.

[#4]

Ok Mav, thank you