FTTC Issue

So I reported a fault to my ISP talktalk about my connection dipping from around 50mbs to sometimes around 5 at times (I am on the large package 74/20). Sync speed would remain around 55 but the actual download speed would drop.

They sent out engineer no issue in house, openreach guy comes out and tells me there is major issue with underground cables and will need to be fixed. Last wednesday my connection went down completely for around 4/5 hours which I hoped was the issue being resolved but it is still on going. My sync speed has been slowly going down over the past two weeks and as of today it is at 15 down 4 up. I know it is hard to say not knowing what the exact issue is but does this sound normal for the issue being worked on?

Talktalk have been calling to let me know the problem is still being worked on, last call was on Friday, but can't really give me more info other than openreach are dealing with it.

If the sync speed was remaining steady at 55 Mbps and SNR margin had not plummeted then suggestion would be that the issue was nothing to do with local cabling anyway, but maybe a congestion (shared nature of the Internet issue - e.g. if the slow downs are all in the evening i.e. peak time then adds to that theory).

If Openreach has been working on the local cabling as a result of your fault report, then once they mark job as fixed the DLM system should be reset and line will relearn its sweet spot.

So all in all things do not sound normal and without more information about the original issue hard to judge.