My grey cells are on holiday (I'm joining them tomorrow), but I found a nifty little utility that allows you to work out what the "right" MTU is for your system. It worked, too. I'm blowed if I can remember or find it, though. It might even have been on here.
There seem to have been a lot of glitches on a lot of websites and ISPs over the past week - I don't know if it's all coincidence, or if there's something new in the world of hackers and trolls.
I noticed earlier this week that a website I connect to regularly had turned very slow and unreliable. A traceroute showed that the normal "my ISP - Telehouse - the site" had added an extra step that went via a site in Italy. Fishy, that, I thought. Now, the Italian loop has gone again, and it's working normally.