Can anyone suggest an answer to this mystery?

I am a Talktalk customer using a FTTC connection. My local exchange is Usk. From 17 October 2020 until January 2021 I was experiencing an outage ALMOST every weekday. My modem router, the Talktalk Sagemcom model, would 'freeze' and internet connection would be lost until I reset it by powering down the router and turning it on again.
Talktalk sent me three different Sagemcom Routers.
The frozen lines was almost always in the working day; only once on a Saturday, otherwise on weekdays. Twice I had Openreach engineers to check the line; the first time the line was clear but the engineer changed my master socket front to a two outlet socket (phone and data). The second time the engineer told me that I was the tenth Talktalk customer she had visited that day with identical problems but the lines always tested perfectly. She told me that there were 41,000 Talktalk customers having the same problem...
Now my dentist, on Plusnet, from the same exchange but a different cabinet seems to be having a similar problem with regular weekday outages.
My line is now stable (fingers crossed) ever since Talktalk changed my modem from a Sagemcom one to a Huawei DG8041W - which looks identical.
I find it hard to understand why three Sagemcom routers froze but this Huawei one doesn't and what was causing the freezes? Was it the router at all, or just conincidence that the fault was cured at the same time as I got the new kit. I am curious and wonder if anyone might have a possible answer!

Maybe a bug in the Sagecom router firmware. I used to have an ADSL router with a similar problem. My ISP at the sime sent me a replacement router which was just as bad. Then I became a Plusnet customer and one day I noticed there as new firmware for the router on the Plusnet site. I downloaded and installed this firmware and the router workd perfectly afterwards.

The is quite possible, but what puzzles me is the less-than-random nature of the outages.
Always during the working day, never - except once - at weekends.
Would a firm-ware fault show up like this, I wonder?

Actually come to think of it, Talktalk claimed they were updating my firmware from their end a few weeks after I received the third of the of the Sagemcom routers. After which the outages were less regular.

So that could be a factor - perhaps a combination of factors.

Thank you for your suggestion Mr Chare