Could i ask why after the first bad experience you went back when when as you say "Both times after leaving TalkTalk, the issue disappeared" did you have connection problems with the new provider and decide to give TT another chance?
I move around occasionally to find good deals, try different ISPs and whatever hardware they supply, out of curiosity more than anything else. I've had infrequent line issues while on other ISPs but Openreach have always fixed it without me having to change ISP (joint corrosion).
I returned to TT because I was baffled by the incident the first time, and my assumption was that it was a hardware issue on TT's exchange hardware and that whatever issue was causing it was fixed in the intervening 4 years. Evidently it wasn't.
Literally my next door neighbour has months of uninterrupted uptime on TT. Having been around these forums and having dealt with REIN issues a long time ago, I'm still at a loss as to what was causing it.
Incidentally the second time I left TT is wasn't just a broadband drop, any active voice call would drop too. That would almost certainly rule out REIN, even if the chances of developing severe REIN just after joining TT, and losing severe REIN as soon as I left were extremely unlikely anyway. I never tested for voice drops the first time as the drops were more random.
I find it a bit frustrating really, as I think TT are a good value ISP and I like their latest router (which I still have) so I would have happily stayed with them both times were it not for this issue.