>>> I suppose it's possible that your PPP session was disconnected & you ended up on a different gateway (slower/more congested).
Most 'normal' people wont notice something like that.. but the more astute users (and avid gamers) do.
BT's routing can sometimes be weird... there have been times in the past when I was on a BTw line I wondered if I was routed from here then back up to Glasgow ane then down to London because of jumps in latency between the RAS and destination. You used to be able to ping the RAS, but I dont think you can now.. this used to be a pretty helpful diagnostic, because it seemed to happen from my exchange a couple of times a year.
For some reason the graphs gone now.. but I did see it earlier... the latency looked pretty static and from what I can remember, not really like anything what you'd expect with congestion where it tends to spike all over the show.
If interleaving/error correction on the DLM isnt the cause, then it is possible to be a change of routing :/
If so its not always easy to pick up new routing.. you can try leaving the router off overnight, but it can sometimes be the backhaul routing between the exchange and RAS, in which case you may be stuck with it for a few days/weeks