speed issues

We have an issue on a site which has had decent enough broadband service for the longest time. Over the past couple of weeks people have been complaining about "slow internet". It's got quite ratty the last few days.
I've run a lot of speed tests, I've tried different routers to ensure it's not just that.
This is on the Erskine exchange.

The router is showing SNR margin of 12 (11,12,13,14) and Attenuation around 48.
It was showing SNR around 4 until the provider changed that a few days ago.
The router is a Vigor 2820Vn

Typical speeds the last few days are around 4500 Kbits download, and almost always 138 Kbits upload. The router sync is showing 5208000 download and 190400 upload.

When it was showing SNR around 4 , the speedtests were returning wide variations between 2000 and 6800 download , and fairly constant around 220 upload. Router sync at that time was generally around 6912000 - 7360000 download while the upload varied a lot, 256000 to 416000

Does anyone have any words of wisdom on these results ?
When I've tried another modem and another router, the upload speed differs tremendously.

In reply to a post by clyde123:

I've run a lot of speed tests, I've tried different routers to ensure it's not just that.

When I've tried another modem and another router, the upload speed differs tremendously.

? Those statements seem to be a bit incompatible. Also the actual different figures down and up I feel are needed.

The full line stats from the Vigor and the better upload modem/router would be best. Please copy and paste them, as the figures you give don't seem valid.

In particular the upstream sync of 190.4kbps.

Why did the ISP up the target noise margin, and what is that margin immediately after a reconnection please? (So if you take those full line stats at that point, there's only one set of readings per modem/router).

If it is a BT Wholesale based line then it was probably automatic by the system, due to something bad happening to the line. If it is LLU the implication is a report by a user of instability. Again, the name of the ISP and the product name is far more likely to help us with a diagnosis than "the provider" . Different providers have different systems, and because of that some have issues are well-known and relatively easily corrected.